Use @project as default on ToggleSubscriptionAction concern

app/controllers/concerns/toggle_subscription_action.rb

@@ -12,7 +12,7 @@ module ToggleSubscriptionAction
   private
 
   def subscribable_project
-    raise NotImplementedError
+    @project || raise(NotImplementedError)
   end
 
   def subscribable_resource

app/controllers/projects/issues_controller.rb

@@ -169,10 +169,6 @@ class Projects::IssuesController < Projects::ApplicationController
   alias_method :awardable, :issue
   alias_method :spammable, :issue
 
-  def subscribable_project
-    issue.project
-  end
-
   def authorize_read_issue!
     return render_404 unless can?(current_user, :read_issue, @issue)
   end

app/controllers/projects/labels_controller.rb

@@ -128,10 +128,6 @@ class Projects::LabelsController < Projects::ApplicationController
     @available_labels.find(params[:id])
   end
 
-  def subscribable_project
-    @project
-  end
-
   def find_labels
     @available_labels ||= LabelsFinder.new(current_user, project_id: @project.id).execute
   end

app/controllers/projects/merge_requests_controller.rb

@@ -473,10 +473,6 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   alias_method :issuable, :merge_request
   alias_method :awardable, :merge_request
 
-  def subscribable_project
-    merge_request.project
-  end
-
   def authorize_update_merge_request!
     return render_404 unless can?(current_user, :update_merge_request, @merge_request)
   end