Replace wysihtml5 editor with trumbowyg/froala

[roadsideseb]

• 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.