Use the ajax-key session-auth thing (fixes #1).

Since the addition of cburschka/ejabberd-auth-php#1
has allowed creating a login key for ejabberd based
on a session cookie on an arbitrary PHP site (only SMF
for now), cadence now has a feature to use it.

Remaining tasks:
 * Actually make the relevant settings key visible in
   the settings interface.
 * Allow reconnecting with an empty connect command,
   to avoid having to reload the page when the connection
   is lost.