Replace usage of document.execCommand (marked as obsolete)

MDN has marked document.execCommand as obsolete. We use it in our WYSIWYG editor and also to copy/paste in desktop/browser.

We should remove this code and replace it with an alternative. The bad news is that there is no easy alternative, from what I've seen we're supposed to work with Range&Selection and implement this feature ourselves.

TrixEditor, made by Basecamp, announces that they don't use execCommand in their editor. I took a look if we could use it in the app, maybe it would save us some trouble, but I didn't see any Angular2 adapter so we'd have to do it ourselves.