Move the #toggle_subscription controller method to a concern

app/controllers/projects/issues_controller.rb

 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!
@@ -110,14 +112,6 @@ class Projects::IssuesController < Projects::ApplicationController
     redirect_back_or_default(default: { action: 'index' }, options: { notice: "#{result[:count]} issues updated" })
   end
 
-  def toggle_subscription
-    return unless current_user
-
-    @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
@@ -131,6 +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)

app/controllers/projects/labels_controller.rb

 class Projects::LabelsController < Projects::ApplicationController
+  include ToggleSubscriptionAction
+
   before_action :module_enabled
-  before_action :label, only: [:edit, :update, :destroy, :toggle_subscription]
+  before_action :label, only: [:edit, :update, :destroy]
   before_action :authorize_read_label!
-  before_action :authorize_admin_labels!, except: [:index, :toggle_subscription]
+  before_action :authorize_admin_labels!, only: [
+    :new, :create, :edit, :update, :generate, :destroy
+  ]
 
   respond_to :js, :html
 
@@ -60,13 +64,6 @@ class Projects::LabelsController < Projects::ApplicationController
     end
   end
 
-  def toggle_subscription
-    return unless current_user
-
-    @label.toggle_subscription(current_user)
-    render nothing: true
-  end
-
   protected
 
   def module_enabled
@@ -80,8 +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)

app/controllers/projects/merge_requests_controller.rb

 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]
@@ -233,14 +234,6 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     render json: response
   end
 
-  def toggle_subscription
-    return unless current_user
-
-    @merge_request.toggle_subscription(current_user)
-
-    render nothing: true
-  end
-
   protected
 
   def selected_target_project
@@ -254,6 +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