Image description authoring (rebased)

Rebased version of !594 (closed) with a couple of syntax/style fixes to make Vue happy. There was some mention of !594 (closed) needing UX enhancement, I'm pretty happy with the current UX and feel like getting this long-overdue feature out there in any form should be the priority, unless there are any glaring usability issues that need fixing. I've got this patch deployed on my personal instance and haven't run into any problems yet.