Merge branch '12743-subscribe-to-label' into 'master'

Allow subscribing to labels

This implement #12743 and supersedes !2799 (thanks @timothyandrew!).

- Subscribe (and unsubscribe) for labels
- When an issue/merge request is created, notify all subscribers of all its labels
- When an issue/merge request is edited, notify all subscribers of all its newly-added labels

## Done

- [x] Verify that a user is signed in to subscribe/unsubscribe from a label.
- [x] Merge conflicts
- [x] Integration tests
- [x] `issuable.subscribed?` should check subscribers and participants
- [x] When an issuable is relabeled, notify subscribers of the *newly added* labels only
- [x] Screenshots:

### Labels page

![Screen_Shot_2016-03-07_at_19.05.44](/uploads/3e69064e9e1a06ee2e27c778776c1407/Screen_Shot_2016-03-07_at_19.05.44.png)

### HTML email

![Screen_Shot_2016-03-07_at_19.07.36](/uploads/e484e06366a738385f1f6d71b52eecf7/Screen_Shot_2016-03-07_at_19.07.36.png)

### Plain text email

![Screen_Shot_2016-03-07_at_19.07.50](/uploads/dc05f710a8f7ab5eae9f72aa2110e741/Screen_Shot_2016-03-07_at_19.07.50.png)

PS: I've set the milestone to 8.6 since it's getting late for 8.5...

See merge request !3115

4 files changed, 31 insertions, 16 deletions

diff --git a/app/controllers/concerns/toggle_subscription_action.rb b/app/controllers/concerns/toggle_subscription_action.rb
new file mode 100644
index 00000000000..8a43c0b93c4
--- /dev/null
+++ b/app/controllers/concerns/toggle_subscription_action.rb
@@ -0,0 +1,17 @@
+module ToggleSubscriptionAction
+  extend ActiveSupport::Concern
+
+  def toggle_subscription
+    return unless current_user
+
+    subscribable_resource.toggle_subscription(current_user)
+
+    render nothing: true
+  end
+
+  private
+
+  def subscribable_resource
+    raise NotImplementedError
+  end
+end
diff --git a/app/controllers/projects/issues_controller.rb b/app/controllers/projects/issues_controller.rb
index 67faa1e4437..b0a03ee45cc 100644
--- a/app/controllers/projects/issues_controller.rb
+++ b/app/controllers/projects/issues_controller.rb
@@ -1,6 +1,8 @@
 class Projects::IssuesController < Projects::ApplicationController
+  include ToggleSubscriptionAction
+
   before_action :module_enabled
-  before_action :issue, only: [:edit, :update, :show, :toggle_subscription]
+  before_action :issue, only: [:edit, :update, :show]
 
   # Allow read any issue
   before_action :authorize_read_issue!
@@ -110,12 +112,6 @@ class Projects::IssuesController < Projects::ApplicationController
     redirect_back_or_default(default: { action: 'index' }, options: { notice: "#{result[:count]} issues updated" })
   end
 
-  def toggle_subscription
-    @issue.toggle_subscription(current_user)
-
-    render nothing: true
-  end
-
   def closed_by_merge_requests
     @closed_by_merge_requests ||= @issue.closed_by_merge_requests(current_user)
   end
@@ -129,6 +125,7 @@ class Projects::IssuesController < Projects::ApplicationController
                  redirect_old
                end
   end
+  alias_method :subscribable_resource, :issue
 
   def authorize_update_issue!
     return render_404 unless can?(current_user, :update_issue, @issue)
diff --git a/app/controllers/projects/labels_controller.rb b/app/controllers/projects/labels_controller.rb
index ecac3c395ec..40d8098690a 100644
--- a/app/controllers/projects/labels_controller.rb
+++ b/app/controllers/projects/labels_controller.rb
@@ -1,8 +1,12 @@
 class Projects::LabelsController < Projects::ApplicationController
+  include ToggleSubscriptionAction
+
   before_action :module_enabled
   before_action :label, only: [:edit, :update, :destroy]
   before_action :authorize_read_label!
-  before_action :authorize_admin_labels!, except: [:index]
+  before_action :authorize_admin_labels!, only: [
+    :new, :create, :edit, :update, :generate, :destroy
+  ]
 
   respond_to :js, :html
 
@@ -73,8 +77,9 @@ class Projects::LabelsController < Projects::ApplicationController
   end
 
   def label
-    @label = @project.labels.find(params[:id])
+    @label ||= @project.labels.find(params[:id])
   end
+  alias_method :subscribable_resource, :label
 
   def authorize_admin_labels!
     return render_404 unless can?(current_user, :admin_label, @project)
diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 03ba289eb94..61b82c9db46 100644
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/app/controllers/projects/merge_requests_controller.rb
@@ -1,10 +1,11 @@
 class Projects::MergeRequestsController < Projects::ApplicationController
+  include ToggleSubscriptionAction
   include DiffHelper
 
   before_action :module_enabled
   before_action :merge_request, only: [
     :edit, :update, :show, :diffs, :commits, :builds, :merge, :merge_check,
-    :ci_status, :toggle_subscription, :cancel_merge_when_build_succeeds
+    :ci_status, :cancel_merge_when_build_succeeds
   ]
   before_action :closes_issues, only: [:edit, :update, :show, :diffs, :commits, :builds]
   before_action :validates_merge_request, only: [:show, :diffs, :commits, :builds]
@@ -233,12 +234,6 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     render json: response
   end
 
-  def toggle_subscription
-    @merge_request.toggle_subscription(current_user)
-
-    render nothing: true
-  end
-
   protected
 
   def selected_target_project
@@ -252,6 +247,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   def merge_request
     @merge_request ||= @project.merge_requests.find_by!(iid: params[:id])
   end
+  alias_method :subscribable_resource, :merge_request
 
   def closes_issues
     @closes_issues ||= @merge_request.closes_issues