Add ability to edit sent messages

[cauebs]

People make mistakes. Let's help users undo them.

Suggested experience:

• an "edit" icon (similar to the ones for Mute and Permalink), but positioned right next to the timestamp, which enables editing
• pressing the up arrow while in the text box, which enables editing; escape cancels

[rafd]

We're bringing back issues on the main repo, because it's more natural for folks to submit that way. Since this issue is still relevant, I'm re-opening it.

[rafd]

Deleting a message is now implemented. Updating this issue to be just about editing messages.