Replace wysihtml5 editor with trumbowyg/froala

* Remove the old rich text editor 'wysihtml5'.
* Generalize the markup to hook up rich text editor into block edit
  forms.
* Provide customizable backbone views for rich text editor for
  initializing the editor and enabling live updates.
* Provide default implementation for Trumbowyg editor.
* Add implementation for Froala editor (not including requirements due
  to licensing terms).
* Convert related browser tests to function-based tests and make them
  run with the new setup.

Resolves #49.