Marky fixes and refreshment

[denyshon]

Resolves mozilla/sumo#3221, resolves mozilla/sumo#3230, resolves mozilla/sumo#3231, resolves mozilla/sumo#3232, resolves mozilla/sumo#3234, resolves mozilla/sumo#3235, resolves mozilla/sumo#3233, resolves mozilla/sumo#3236.

• Update the JS code for Marky (switch from prototypes to classes, var -> const/let, etc.).
• Drop old IE-specific code (IE won't be able to run the updated code anyway).
• Split SimpleButton into two classes: MarkupButton (which keeps the old behavior) and InsertButton (which replaces the selection instead of framing it).
• LinkButton, MediaButton, CannedResponsesButton and QuoteButton now inherit from InsertButton, which also contains some shared code.
• Unify selection creation for codemirror and regular textarea.
• Switch LinkButton kbox to and make the link fields required (link type must be selected, and a link for this type must be specified). Disable the Insert button until the form is valid.
• Add a missing </div> in the LinkButton kbox html template (its absence led to an extra margin at the bottom).
• Allow submitting links without text.
• Other fixes and improvements.

[denyshon]

I'll need to rewrite this since #7718 has been merged