Show & autocomplete PM recipient names & emails

[neiljp]

Currently only emails are shown in PM 'to' fields when sending a message. The server is moving towards hiding emails optionally or fully (preferring user-ids, see #579), but even if this was not the case, while showing email addresses is unique, email addresses don't always easily translate to an obvious name.

The simplest (if verbose) solution to this would be to do something like in email, ie. Some Person <[email protected]>. We should discuss others.

Any user_id changes underneath could be separate.

[neiljp]

Stream/topic work is mostly complete as-of #746, so improving PM input/display is a high priority.

[neiljp]

See #586 for more discussion on this.

[prah23]

I see that this hasn't been worked on yet, I'd like to take this up.

[neiljp]

@prah23 You're welcome to take this up 👍 It would be good hear your plans for the UI.