cecfa724aa
Currently, we don't add users to the attention set when some events occur, e.g adding or removing reviewers, replying on comments, etc. However, it makes sense that some events cause the change to no longer require attention from anyone. In those cases, we should remove all users from the attention set. Those cases are: closing a change, and marking work in progress. Change-Id: I645219c1a928f34e5eace688806e39e748cfcfae