Add CONFERENCE property to events linked to Talk rooms

[tcitworld]

I tried that, there is a problem with the encoding of FEATURE param in ical.js
Have to fix / report that upstream. :)

Originally posted by @georgehrke in #1732 (comment)

Also see CONFERENCE Spec: https://datatracker.ietf.org/doc/html/rfc7986#section-5.11

use this for saving the Talk Room URL and use this info to show a "join-button" on the:

• calendar event (hover)
• reminder/announcement list as an actionable button
  if the URL matches the pattern of a talk room, could also work for other link types, like protocolls MS-Teams, Skype, Telephone number