Add validation for signin/signup

Closes #2976, Closes #3017

- 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
- Modify ValidationEngine.validate to not format errors based on option
- Update casper test for signin to wait for notification before trying
to do another signin
Merge pull request #3051 from jgable/loginValidation

Add validation for signin/signup
Ensure incomplete tags aren't lost on save

closes #2991, references #2172, references #2453
- moved and separated tags logic from EditorTagsView into PostTagsInputController/View
- call out to PostTagsInputController when saving post to ensure an incomplete tag is completed before save
- added Tab key support for suggestion selection / tag completion
- don't show suggestions list when input field doesn't have focus
- added code for #2172 but left commented-out as it causes side effects with completion on save
- updated suggestion highlighting so it doesn't bork on html/regex chars (#2453)