This uses our own gomatrix lib with the SendHTML function which
adds HTML to formatted_body in matrix.
golang-commonmark is used to convert markdown into valid HTML.
An extra avatarMap (cache) is created for mattermost and telegram.
If MediaServerUpload is configured, the avatar images of users are downloaded the first time a
user sends a message.
If this download succeeds a message with EVENT_AVATAR_DOWNLOAD is sent to the originating protocol.
This message also contains a SHA field (in msg.Extra["file"]), if this is not empty, the sha will
be added to the avatarMap. (so we now have a userid-sha cache)
Next time this user sends a message, the MediaServerUpload/sha/userid.png URL will be used as the
avatar field.
