Multi-tag selection on resource listings

In #241 we introduced a resource page, and @Bisaloo requested the following:

> I would love to have the ability to select a combination of tags. E.g., "testing" and "APIs", which would make it easier to find resource such as https://books.ropensci.org/http-testing/index.html.

This is a great idea! Opening this issue to track work on this and not expand the scope of #241.

Start by examining the resource page introduced in #241 and its existing tag-selection behavior. Verify how multiple selected tags should combine, then confirm that selecting tags such as “testing” and “APIs” shows resources matching the combination and that the single-tag behavior remains intact.

Written by the indexing model from the issue text.