R1:aacc62463d61

Prevent editing and deleting comments in locked conversations

Summary:
Ref T13289. This tightens up a couple of corner cases around locked threads.

Locking is primarily motivated by two use cases: stopping nonproductive conversations on open source installs (similar to GitHub's feature); and freezing object state for audit/record-keeping purposes.

Currently, you can edit or remove comments on a locked thread, but neither use case is well-served by allowing this. Require "CAN_INTERACT" to edit or remove…