Add an ical (.ics) feed that includes all our events

[BudgieInWA]

This PR adds a template that generates an .ics file with our events in it, closing #53.

It also improves the look of the events list on the site.

[BudgieInWA]

I have tested this by manually importing the events into Google Calendar and it's working well: The title, start time, end time (or duration), location and description are all coming through.

Google calendar would parse html out of DESCRIPTION (where it shouldn't be) but is ignoring X-ALT-DESC, which is a Microsoft thing that I haven't tested yet.

I haven't tested it as a subscribed feed, so I don't know yet if modifications to the event details will flow through to clients. The DTSTAMP field should help with that. (Note to self: check if the timezone shenanigans works on GH Pages). One neat addition would be populating LAST-MODIFIED using https://github.com/gjtorikian/jekyll-last-modified-at, but we would have to build the site ourselves (or use GH Actions) to use arbitrary plugins, as far as I can tell.