Move the handling of chat events into the shared library so it can be
used by multiple recording formats.
The anonymization of names is based on the external user id, if
available, so users have a consistent name through the meeting. Note
that no effort is made to edit chat messages - if someone is mentioned
by name in a chat message, that will still be visible.
Default settings for anonymization can be controlled in
bigbluebutton.yml, and per-meeting overrides can be done using meta
parameters on the create call.
