Original implementation to allow users to subscribe to labels

1. Allow subscribing (the current user) to a label

- Refactor the `Subscription` coffeescript class
  - The main change is that it accepts a container, and conducts all
    DOM queries within its scope. We need this because the labels
    page has multiple instances of `Subscription` on the same page.

2. Creating an issue or MR with labels notifies users subscribed to those labels

- Label `has_many` subscribers through subscriptions.

3. Adding a label to an issue or MR notifies users subscribed to those labels

- This only applies to subscribers of the label that has just been
  added, not all labels for the issue.

2 files changed, 21 insertions, 4 deletions

diff --git a/app/mailers/emails/issues.rb b/app/mailers/emails/issues.rb
index 4a88cb61132..2838baa1b4e 100644
--- a/app/mailers/emails/issues.rb
+++ b/app/mailers/emails/issues.rb
@@ -16,7 +16,14 @@ module Emails
     def closed_issue_email(recipient_id, issue_id, updated_by_user_id)
       setup_issue_mail(issue_id, recipient_id)
 
-      @updated_by = User.find updated_by_user_id
+      @updated_by = User.find(updated_by_user_id)
+      mail_answer_thread(@issue, issue_thread_options(updated_by_user_id, recipient_id))
+    end
+
+    def relabeled_issue_email(recipient_id, issue_id, updated_by_user_id, label_names)
+      setup_issue_mail(issue_id, recipient_id)
+      @label_names = label_names
+      @updated_by = User.find(updated_by_user_id)
       mail_answer_thread(@issue, issue_thread_options(updated_by_user_id, recipient_id))
     end
 
@@ -24,7 +31,7 @@ module Emails
       setup_issue_mail(issue_id, recipient_id)
 
       @issue_status = status
-      @updated_by = User.find updated_by_user_id
+      @updated_by = User.find(updated_by_user_id)
       mail_answer_thread(@issue, issue_thread_options(updated_by_user_id, recipient_id))
     end
 
diff --git a/app/mailers/emails/merge_requests.rb b/app/mailers/emails/merge_requests.rb
index 325996e2e16..680ce975c79 100644
--- a/app/mailers/emails/merge_requests.rb
+++ b/app/mailers/emails/merge_requests.rb
@@ -19,10 +19,20 @@ module Emails
                          subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
     end
 
+    def relabeled_merge_request_email(recipient_id, merge_request_id, updated_by_user_id, label_names)
+      setup_merge_request_mail(merge_request_id, recipient_id)
+      @label_names = label_names
+      @updated_by = User.find(updated_by_user_id)
+      mail_answer_thread(@merge_request,
+                      from: sender(@merge_request.author_id),
+                      to: recipient(recipient_id),
+                      subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
+    end
+
     def closed_merge_request_email(recipient_id, merge_request_id, updated_by_user_id)
       setup_merge_request_mail(merge_request_id, recipient_id)
 
-      @updated_by = User.find updated_by_user_id
+      @updated_by = User.find(updated_by_user_id)
       mail_answer_thread(@merge_request,
                          from: sender(updated_by_user_id),
                          to: recipient(recipient_id),
@@ -42,7 +52,7 @@ module Emails
       setup_merge_request_mail(merge_request_id, recipient_id)
 
       @mr_status = status
-      @updated_by = User.find updated_by_user_id
+      @updated_by = User.find(updated_by_user_id)
       mail_answer_thread(@merge_request,
                          from: sender(updated_by_user_id),
                          to: recipient(recipient_id),