Events: Make filter URLs pretty

[iandunn]

Fixes #1196

WIP

When finished, applying filters should result in a URL like https://events.wordpress.test/upcoming-events/filtered/format/online/ and
https://events.wordpress.test/upcoming-events/filtered/format/in-person/type/wordcamp/month/january,february/country/united-states,germany/. Currently that's only partially working.

When finished, you should also be able to start at https://events.wordpress.test/upcoming-events/ and apply filters to get to a "pretty" URL like the one above, rather than the key-value pairs shown in #1196. This part will probably require some modification to the query-filters block, to append the selected filters to the form action in JS.

[iandunn]

I setup a rewrite rule so that URLs like https://events.wordpress.test/upcoming-events/filtered/format/online/ will be treated the same as https://events.wordpress.test/upcoming-events/?format_type=online. That part works.

The other main part of this is to redirect the user to that type of URL when they select filters in the query-filters block. That currently isn't working, the filters still get added to the URL as encoded key-value pairs, because they're being created as input fields.

One solution would be so enqueue some JS from this theme, and update the <form action="..."> on the fly when the user selects a value from an instance of the block, and also remove the <input> fields from the form on submit. That doesn't feel very elegant, though.

Another solution would be to modify the query-filters block directly, but I think that'd add tech debt.

This is related to previous work around treating the arbitrary event data work as WP_Query results. See #1124 and #1156.

@StevenDufresne , @ryelle: i'm curious what you think about this.

[iandunn]

Ah, I see now that Showcase submits to the URL with key-value pairs, then redirects to the "pretty" one. That should work here too 👍🏻