Support for custom edit summaries

Current behavior: <autocomment> <usersummary>|<autosummary>, means
always display the autocomment and replace the autosummary with a
user-provided summary, if given

Bug: 41490
Bug: 50307
Bug: 46537
Bug: 49527

Change-Id: Ia8f988734f69043cb4214653bbf109f4bf271c60
Merge "Support for custom edit summaries"