Guarantee unique resource and more

This is mostly a collection of little fixes and clean-ups.  The main one is to make sure that the default XMPP Resource is always unique by moving that code into `Imps.getXmppResource()`.

The other key one is an update of the Certificate Authority pins.  This includes all CAs used by the recommended services so they do not show the MemorizingTrustManager TOFU dialog.