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 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.

Test Plan:

• Changed the description of a task via bulk editor.
• Added a comment to a task via bulk editor.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13025, T12415

Differential Revision: https://secure.phabricator.com/D18867