Allow blocking reviewers to be added via the web UI
Summary:
Ref T10939. Adds a blocking(...) token.
This code is pretty iffy and going to get worse before it gets better, but the fix (T10967 + EditEngine) is going to be a fair chunk of work down the road.
Test Plan:
Reviewers: chad
Reviewed By: chad
Subscribers: scode
Maniphest Tasks: T10939
Differential Revision: https://secure.phabricator.com/D15933