Improve gathering support

[soapdog]

This has been outlined originally in the new funding proposal of 2021: %Y2iyBckEhcWvxX6a1+9ELeP6wFYD80QNdhvR50V1C7E=.sha256.

Improve gathering support
At the moment, you can view gatherings, export them to your calendar application, and mark yourself as attending them. The missing feature is actually creating gatherings of your own. I’d like to improve many aspects of the gathering support, but most crucially, I’d like the user to be able to create new gatherings (and maybe even import from iCal files).