add endpoint for getting one event as ICS

[MoralCode]

RIT's calendar download files have been broken for ages (re: multi line descriptions), so we should offer ones that function

We need a way to uniquely identify each event to faciliitate the download. other than that, theres already the ability to create (compliant!) ICS files

[MoralCode]

This also slightly depends on ensuring the scraper actually saves all the info so the downloads largely match the same event recurrences as the original: CampusPulse/event-data-ingest#8