Fix: Store RSVP locations as a `List<String>`

I didn't notice until the other day that RSVPs via meetup-mf2 were rendering with an invalid location.

It appears that this is due to the response from meetup-mf2 coming back correctly as a String[], but being put into my site as a String.

This was originally worked on in meetup-mf2!5 (merged) but I appear to have overlooked it until now within this project.

We shouldn't mangle whatever is there, but instead store it in an MF2-like format.

As part of jvt.me#925.