Modernize transaction comment editor

Summary: Fixes T8703. The URI handling here was a little unusual.

Test Plan:

• Edited and deleted comments in several applications, including Macro.
• As an admin, deleted others' comments.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8703

Differential Revision: https://secure.phabricator.com/D13469