Redesign the action buttons toolbar for the realtime WYSIWYG editor

Now that realtime WYSIWYG editing is enabled by default we need to rethink a bit the action buttons toolbar, to see which buttons / controls still make sense:

• with auto-save it's probably useless, and even confusing to show the Cancel button
• with auto-save the two Save buttons could be replaced by a single button ("Close"?) that goes back to view (and saves)
• we may want to make the summary input and the "Minor edit" checkbox less visible
• we might want to show the other users that are editing, in order to remove the double toolbar from CKEditor, and to prepare for realtime inline forms where it doesn't make sense to show the editing users for each widget but for the entire form, on the action buttons toolbar.