Keyboard shortcut for joining a channel (shift-j).

This also slightly tweaks new channel interactions in general:

- Clicking away from the new channel dialog hides it
- The dialog retains your text in case you open it again
- Hitting esc also hides this dialog
- Adding a new channel or closing the dialog doesn't leave you in a
  state of non-working keyboard shortcuts
Joining channel now switches to it.
Merge pull request #75 from bkad/add-new-channel-ux-fixes

Better UX for 'add new channel' feature