New permission/ACL for toggling message editing

Problem

Users are able to edit messages after sending them in channels or communities where that is not desired.

Proposal

I propose that there should be a role permission line item for "edit message", just like the "Send messages" line item. image

Notes

Some contexts require write-once participation. Other use cases of this feature would be to prevent "edit trolling" where a user posts one thing, gets responses, then edits their message post-hoc to something entirely different.