Rename config.sessionName to config.clientName.

The next patch is going to reuse this string for generating
the JID resource identifier, so it should have a generic name.
Fix #108: Make JID resource configurable (hidden setting).

New settings key: config.settings.xmpp.resource
 default value is {client}/{version}/{timestamp}.

 * {client} becomes config.clientName ("cadence")
 * {version} becomes config.version
 * {timestamp} becomes (new Date()).getTime()

The settings key is not exposed in the UI, but can be set in the JS
console. To force a save, it is enough to toggle any sidebar.