fix bug 921150 - Modify WYSIWYG scrolling behavior

So I mostly followed what was request in the ticket but we can't eliminate the scrolling behavior in the translation interface. Anyways, here's what this does:

On the "Edit" and "New" document pages, the editor is sized to the user's initial screen, so there's no scrolling behavior.

On the "Translate" interface, I've simplified the existing code. It brings a tear to my eye.

Waffled as @openjck requested.

Supercedes https://github.com/mozilla/kuma/pull/1449