User should be able to edit (or hide) their own messages

[holmesworcester]

Users should be able to edit their own messages, and all other users should see the updated message, with an "(edited)" note after the message, like slack.

On desktop, up arrow in an empty message field should edit the last message.

Design for desktop and mobile here: https://www.figma.com/file/y8h6w8PYR9jyI3zjYHL9Cl/Mobile-%2B-desktop-%2B-prototypes?node-id=1927-44767&t=vBWoIOqhhRRYPBDj-4

They should be able to hide messages too, as a quick alternative to deletion that doesn't require backend changes.