Add validation for signin/signup

Closes #2976

- Move logic to signup controller
  - Add ValidationEngine mixin to signup controller
  - Add signup validator with code from clientold login view
- Add signin validator and integrate into signin controller
- Add validation to forgotten controller
  - Switch to button action to support hitting enter in text field to submit
- Clear all notifications in notifications.closeAll
I really don't think this text is supposed to change. I believe it the test is failing, either because the test is now wrong because the button is being disabled as you mentioned on IRC, or because something else is wrong.

The 'blah attempts remaining' text is what is supposed to appear when your email & password combo is incorrect, which should not appear under these test circumstances, I think?