Depends on D18866. Ref T13025. Fixes T12415. This makes the old "Add Comment" action work, and adds support for a new "Set description to" action (possibly, I could imagine "append description" being useful some day, maybe).
The implementation is just a <textarea />, not a whole fancy remarkup box with [Bold] [Italic] ... buttons, preview, typeaheads, etc. It would be nice to enrich this eventually but doing the rendering in pure JS is currently very involved.
This requires a little bit of gymnastics to get the transaction populated properly, and adds some extra validation since we need some code there anyway.