Bitbucket Server importer: import declined MR events

Summary

Import events when merge requests are declined on Bitbucket Server.

How

• Add a check to Activity Representation for DECLINED events: https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/bitbucket_server/representation/activity.rb
• Import declined events similar to approved events: https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/bitbucket_server_import/importers/pull_request_notes_importer.rb#L31