XmppUriHelper.parse() parses xmpp:, xmpp://, and xmpp:/// URLs

Parse xmpp URIs according to the RFC5122 and XEP-0147, but currently all
that ChatSecure understands is how to add a contact.  This parser will
parse the non-standards-compliant xmpp:// URL that ChatSecure used on both
iOS and Android to work around iOS's issue with xmpp: URLs.

refs #4092 https://dev.guardianproject.info/issues/4092
handle all incoming xmpp: URIs using XmppUriHandler.parse()

This currently just allows for adding a contact, but xmpp: URIs can do lots
of things, including join group chats.
tweak format of xmpp: URL to work on more apps and follow RFC5122

The format of the xmpp: URL in the QR Code did not follow the standard, and
therefore only worked in ChatSecure.  Apparently, iOS does not respect the
plain xmpp: URL so we have to work around within the RFC5122 standard.
Using the triple / aka blank "authority" follows the RFC and will hopefully
work on iOS.  Unfortunately, many apps don't parse xmpp:/// URLs correctly.

refs #4092 https://dev.guardianproject.info/issues/4092