Better UX for 'add new channel' feature

While it does have a swanky spinning + -> x that makes up for just about anything else, there are a few issues with this guy that could use a bit of polish:

* When you open the dialog and then defocus, it could disappear instead of hanging around (especially if there's no text there).
* Hitting 'esc' should dismiss it
* After you've hit 'enter' and added a new channel, you're in a weird mode where kb shortcuts don't work until you manually focus some area of the chat
* When you add a new channel, should we select it as well? I think this might be the common case.
* (Improvement) It would be nice to give the user a hint that they've already added a channel (say, temporarily highlight it) instead of just doing nothing when they try to add a duplicate.
* (Improvement) Keyboard shortcut to add a channel.

**Edited to add some items.**