Secret API keys for meetings

[edemaine]

• [x] Each meeting should have an automatically generated UUID "API key" that enables superuser-level access.
• [x] Mentioned on Welcome screen, and presented on Settings screen (or option to enter it there if you don't have one), behind "spoiler tag" so it's not accidentally revealed
  • [x] Should also add URL of current meeting to that page, for easier sharing
  • [x] Copy to clipboard button
• [x] Saved in localStorage
• [ ] Required for API calls (#131)
• [ ] Required for presence API call (#104)

Will enable many of the blocked issues, now admin issues and additional things like:

• [ ] Setting default sort for a meeting
• [ ] Setting default Jitsi, Cocreate, etc. servers