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.
