Replace Facebook share button with a more versatile share menu

[carrythebanner]

The two main ways to share an event are:

• Copy the "sharable" link
• Use the Facebook share button

Sharing to FB isn't necessarily the most common action, but it's visually prominent. Also, copying the "sharable" link isn't a super obvious action, compared to more obvious "copy link to this post" features available on many sites.

It would be more versatile if we hooked into the native share functionality. This would let the user send an event to any sharing service on the user's device: email, Airdrop, SMS, notes, etc.

It's not available in all browsers (notably, Firefox) so we'd need a fallback. But, there's plenty of prior art out there for how to handle that, e.g. How to Use the Web Share API — CSS Tricks.

See also #438.