Subscribe to board = auto-subscribe to new topics

* Moved definition of what actions should cause watch (and for what users) to
  FlowActions.
* Created 2 more classes, 1 for immediate & 1 for delayed watching.
* Created job to handle delayed watching.

Also changed exceptions to wfWarn:
* There were issues with the tests, which don't provide
  the for this handler required 'workflow' metadata
* This handler is not "important enough" to warrant an
  exception. If we log failure, that's ok. Don't need to
  halt execution.

Change-Id: I5ed61349c9c19f9ea025d8ab9251c6367dfe6165
Merge "Subscribe to board = auto-subscribe to new topics"