R1:687fada5af43

Restore bulk edit support for remarkup fields (description, add comment)

Summary:
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…