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