XEP-0483: Use XEP-0482 to send meeting URL to invitees

[deleolajide]

This PR is in response to this request

Hi,
This XEP has two parts:

1. An IQ-based protocol to request a meeting link from a server.
2. An additional element to add to a message to send the meeting link
   to another party

The second feature is already available via XEP-0482, which describes
call invites using Jingle and external URIs. The XEP already mentions
weblinks to join the call as an example for such external URIs.
As XEP-0482 has a bunch of other features, like announcing join and
leave on the XMPP side (works especially great if the website is opened
in a frame or similar inside the XMPP client, as the XMPP client then
even knows when the user left the call) or the ability to additionally
announce dial-in tel:-URIs (which some Jitsi instances also support and
may be very useful for mobile clients).

Therefore, I'm proposing to remove the second part from the XEP (or
better: replace it with how to use XEP-0482 for this purpose).

If it is desirable to have a way to signal that the URL is a jitsi URL,
I propose to instead have that as an additional element in the XEP-0482
(which it mentions as a possible extension point
"Specifications that describe further ways to join a call define their
own sub-elements to the element"). This element (e.g. ) would be
additional to the element, so that receiving clients that
don't understand this proposed XEP, but do understand XEP-0482 will
still have a reasonably good UX, just nothing jitsi specific. Replace
jitsi in this paragraph with any other online meeting tool as you see
fit.

Marvin

[CLAassistant]

All committers have signed the CLA.