compose: Support editing a message from any message-list page, not just channel/topic/DM narrow

[chrisbobbe]

Currently, you can edit a message from the message-list page for a channel narrow, a topic narrow, or a DM narrow. This issue is to support editing a message on any message-list page, including the combined feed, the starred-messages page, etc.

This wasn't included in #126 / PR #1498 because some refactoring is needed; the current code is written on the assumption that a given message-list page will either always have or never have a compose box, determined by the Narrow. I think the main thing built on that assumption was safe-area handling, but there may be other things.