Modularize user activity log message types

Summary:
Depends on D20670. Ref T13343. The user activity message log types are currently hard-coded, so only upstream code can really use the log construct.

Under the theory that we're going to keep this log around going forward (just focus it a little bit), modularize things so the log is extensible.

Test Plan:
Grepped for UserLog::, viewed activity logs in People and Settings.

(If I missed something here -- say, misspelled a constant -- the effect should just be that older logs don't get a human-readable label, so stakes are very low.)

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13343

Differential Revision: https://secure.phabricator.com/D20671