[jitsi-dev] Removing XMPP inband registration



I was hoping to quick-fix https://github.com/jitsi/jitsi/issues/136, but this
is more work than expected. The XMPP-Connection in the registration plugin
- use SRV lookup
- ensure a secure connection
- validate the TLS certificate
- change the TLS ciphers
- the list of possible servers is outdated
- the port is field is irrelevant (should come from the SRV lookup)

As I don't have enough time to fix all this, I'll be removing the possibility
of inband-registration of XMPP accounts on Aug. 12th for security reasons
unless someone steps up.