Custom validators

### Changes:
 - Prevents multiple-submit on forms unless there's a validation error. Fixes DoSomething/dosomething#1907.
 - Style "suggestion" validation errors based on "success" value, rather than having a generic "warning" style.
 - Removes special logic I had implemented for "custom validators", instead relying on same validator function logic as everything else.
 - Some other minor cleanups.

For review: @barryclark @weerd