Add caps element to the MUC join presence, so that the MUC MEP node can
know whether we're interested in receiving MEP messages.
Create info messages for any `conference-info` tags that contain `activity` tags.
Check for both `headline` and `normal` MEP messages (even though the XEP
only show `headline` examples), since `normal` messages can be archived
in MAM, but `headline` ones not.
Update the XEP-0372 reference-parsing code to take the `anchor`
attribute into consideration, specifically to check which text element
the reference applies to.
Add support for rendering XEP-0372 mentions in "info" messages and for
triggering HTML5 Desktop notifications for such mentions.
Background:
-----------
XEP-0316 describes a way for a MUC to send out PEP-like messages to MUC
participants. This feature can be used to describe custom activity happening
in the MUC.
Project summary, including supported XEPs in a machine-readable format,
for automated listings, aggregation of XEP implementation status and
other nice things.
See https://xmpp.org/extensions/xep-0453.html
This list was simply scraped from the README and mangled into XML using
`csv2 | sed | 2xml` and amended with a few other details from e.g.
package.json and links on the website.
