Support 'Narrow to current compose box recipient'

[neiljp]

This is useful if starting a new topic - currently we give a notice that the sent message is outside of the current narrow, but there is no easy way of jumping to it.

The web app supports ctrl+. for achieving this, so this is a logical feature for us to also add.

Adding the key should show it in the help menu, but we could also show it in the message that is shown when messages are sent outside of the current narrow.

[zulipbot]

Hello @zulip/server-hotkeys members, this issue was labeled with the "area: hotkeys" label, so you may want to check it out!

[srdeotarse]

@zulipbot claim

[neiljp]

We could also consider a dynamically-shown hint, a little like the UI element being proposed in #design > button to go to conversation.