Add ability to unsubscribe from design management notifications

Currently, it is impossible to unsubscribe from design management
notifications.

Clicking the unsubscribe link in the emails leads to a 404 error.

Changelog: changed

app/models/design_management/design.rb

@@ -14,6 +14,7 @@ class Design < ApplicationRecord
     include Todoable
     include Participable
     include CacheMarkdownField
+    include Subscribable
 
     cache_markdown_field :description
 
@@ -186,6 +187,10 @@ def self.build_full_path(issue, design)
       File.join(DesignManagement.designs_directory, "issue-#{issue.iid}", design.filename)
     end
 
+    def self.to_ability_name
+      'design'
+    end
+
     def new_design?
       strong_memoize(:new_design) { actions.none? }
     end

spec/controllers/sent_notifications_controller_spec.rb

@@ -255,6 +255,26 @@ def unsubscribe
           expect(response)
             .to redirect_to(project_merge_request_path(project, merge_request))
         end
+
+        context 'when unsubscribing from design' do
+          let(:design) do
+            create(:design, issue: issue) do |design|
+              design.subscriptions.create!(user: user, project: project, subscribed: true)
+            end
+          end
+
+          let(:sent_notification) do
+            create(:sent_notification, project: project, noteable: design, recipient: user)
+          end
+
+          before do
+            unsubscribe
+          end
+
+          it 'unsubscribes the user' do
+            expect(design.subscribed?(user, project)).to be_falsey
+          end
+        end
       end
 
       context 'when project is private' do

spec/lib/gitlab/import_export/all_models.yml

@@ -969,6 +969,7 @@ design: &design
 - notes
 - user_mentions
 - events
+- subscriptions
 designs: *design
 actions:
 - design