Implemented new features.

- Icon in the user and group rosters against each user indicating if
they are in an open thread or not.
- When a user or group name is clicked from the roster, a new thread is
opened if one isn’t already. If it is, it is either “hoisted” i.e. made
visible and made active or simply made active if it is visible already.
- Removed Archive tab.
Permit skipping auto-activating thread on open.
Style fixes.

- Stick chat thread to bottom when minimized.
- Remove Minimize button. Clicking on the thread header minimizes it.
Merge branch 'dev' into gh-pages