At some point, BigBlueButton switched from using `"` to `'` in the link tags in
chat messages, which caused the regular expression that was supposed to remove
the `event:` prefix to not match.
I've replaced the error-prone regular expression with an actual HTML parser,
using the "Loofah" HTML transformation/sanitization library based on Nokogiri.
I've removed the code that detected unlinked URLs, since it was broken - and
not needed: current BigBlueButton versions do the link detection in the client.
If someone reprocesses a really old BBB recording with these scripts, URLs in
chat might not be linked in the result. But they wouldn't have been linked in the
client in the original meeting either, so I figure that's ok.
Fixes#6475
