Tab Complete Nicks

This is a rough implementation of autocompleting a nick.
I wasn't sure if `$("form#messageform").on("submit")` is required for other forms of input (mobile) so I left it in. We have to use the `keydown` event instead of the `keyup` event for hooking the tab key, as otherwise it'll perform it's default action (jump to next input).

It only checks the nicks in the backlog right now, and does not fallback to checking the buffer's nick list. I don't think I'll do it the way QuasselClient does it, with it preferring people who you're talking to as that'd be a pain.

I'll probably add in alphabetical completion from the nick list in this PR or in another if you get to this first.