Add comments on changes, for logs and revisions

Differences visible in the UI due to this change:
- Summary field in all logs on Special:CentralNoticeLogs.
- Summary (comment) messages in template and message wiki page
  histories and logs for changes performed via CN admin interface.
- Option to provide a summary message for all modification, creation
  or deletion of Campaigns and Banners via Special:CentralNotice
  and Special:CentralNoticeBanners.

Bug: 31689
Change-Id: I4aaf63dc9a92bbafc299a5b1a0e3ecfa5d682f53
Merge "Add comments on changes, for logs and revisions"