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

3 files changed, 4 insertions, 12 deletions

diff --git a/lib/api/groups.rb b/lib/api/groups.rb
index 9d8b8d737a9..f981ec0dbfe 100644
--- a/lib/api/groups.rb
+++ b/lib/api/groups.rb
@@ -30,7 +30,7 @@ module API
       # Example Request:
       #   POST /groups
       post do
-        authorize! :create_group, current_user
+        authorize! :create_group
         required_attributes! [:name, :path]
 
         attrs = attributes_for_keys [:name, :path, :description, :visibility_level]
diff --git a/lib/api/helpers.rb b/lib/api/helpers.rb
index da4b1bf9902..6a20ba95a79 100644
--- a/lib/api/helpers.rb
+++ b/lib/api/helpers.rb
@@ -129,7 +129,7 @@ module API
       forbidden! unless current_user.is_admin?
     end
 
-    def authorize!(action, subject)
+    def authorize!(action, subject = nil)
       forbidden! unless can?(current_user, action, subject)
     end
 
@@ -148,7 +148,7 @@ module API
     end
 
     def can?(object, action, subject)
-      abilities.allowed?(object, action, subject)
+      Ability.allowed?(object, action, subject)
     end
 
     # Checks the occurrences of required attributes, each attribute must be present in the params hash
@@ -408,14 +408,6 @@ module API
       links.join(', ')
     end
 
-    def abilities
-      @abilities ||= begin
-                       abilities = Six.new
-                       abilities << Ability
-                       abilities
-                     end
-    end
-
     def secret_token
       File.read(Gitlab.config.gitlab_shell.secret_file).chomp
     end
diff --git a/lib/banzai/reference_parser/base_parser.rb b/lib/banzai/reference_parser/base_parser.rb
index 6cf218aaa0d..e8e03e4a98f 100644
--- a/lib/banzai/reference_parser/base_parser.rb
+++ b/lib/banzai/reference_parser/base_parser.rb
@@ -211,7 +211,7 @@ module Banzai
       end
 
       def can?(user, permission, subject)
-        Ability.abilities.allowed?(user, permission, subject)
+        Ability.allowed?(user, permission, subject)
       end
 
       def find_projects_for_hash_keys(hash)