Simplify test and make it run faster

Adding the user to the project via UI makes the test more complex
than needed, and also slower. This commit adds the user to the
project via the API, which is faster and makes the test code
cleaner.

qa/qa/specs/features/browser_ui/2_plan/issue/mentions_spec.rb

@@ -12,12 +12,8 @@ module QA
           resource.name = 'project-to-test-mention'
           resource.visibility = 'private'
         end
-        project.visit!
 
-        Page::Project::Show.perform(&:go_to_members_settings)
-        Page::Project::Settings::Members.perform do |members|
-          members.add_member(@user.username)
-        end
+        project.add_member(@user)
 
         Resource::Issue.fabricate_via_api! do |issue|
           issue.title = 'issue to test mention'