Merge branch 'refactor/add-policies' into 'master'

Refactor ability.rb into Policies

## What does this MR do?
Factors out `ability.rb` into a new abstraction - the "policy" (stored in `app/policies`). A policy is a class named `#{class_name}Policy` (looked up automatically as needed) that implements `rules` as follows:

``` ruby
class ThingPolicy < BasePolicy
  def rules
    @user # this is a user to determine abilities for, optionally nil in the anonymous case
    @subject # this is the subject of the ability, guaranteed to be an instance of `Thing`
    can! :some_ability # grant the :some_ability permission
    cannot! :some_ability # ensure that :some_ability is not allowed. this overrides any `can!` that is called before or after
    delegate! @subject.other_thing # merge the abilities (can!) and prohibitions (cannot!) from `@subject.other_thing`
    can? :some_ability # test whether, so far, :some_ability is allowed
  end

  def anonymous_rules
    # optional. if not implemented `rules` is called where `@user` is nil. otherwise this method is called when `@user` is nil.
  end
end
```

See merge request !5796

2 files changed, 3 insertions, 7 deletions

diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index ebc2a4651ba..bd4ba384b29 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -24,7 +24,7 @@ class ApplicationController < ActionController::Base
 
   protect_from_forgery with: :exception
 
-  helper_method :abilities, :can?, :current_application_settings
+  helper_method :can?, :current_application_settings
   helper_method :import_sources_enabled?, :github_import_enabled?, :github_import_configured?, :gitlab_import_enabled?, :gitlab_import_configured?, :bitbucket_import_enabled?, :bitbucket_import_configured?, :google_code_import_enabled?, :fogbugz_import_enabled?, :git_import_enabled?, :gitlab_project_import_enabled?
 
   rescue_from Encoding::CompatibilityError do |exception|
@@ -97,12 +97,8 @@ class ApplicationController < ActionController::Base
     current_application_settings.after_sign_out_path.presence || new_user_session_path
   end
 
-  def abilities
-    Ability.abilities
-  end
-
   def can?(object, action, subject)
-    abilities.allowed?(object, action, subject)
+    Ability.allowed?(object, action, subject)
   end
 
   def access_denied!
diff --git a/app/controllers/namespaces_controller.rb b/app/controllers/namespaces_controller.rb
index 5a94dcb0dbd..83eec1bf4a2 100644
--- a/app/controllers/namespaces_controller.rb
+++ b/app/controllers/namespaces_controller.rb
@@ -14,7 +14,7 @@ class NamespacesController < ApplicationController
 
     if user
       redirect_to user_path(user)
-    elsif group && can?(current_user, :read_group, namespace)
+    elsif group && can?(current_user, :read_group, group)
       redirect_to group_path(group)
     elsif current_user.nil?
       authenticate_user!