Archives: Update the main heading on all filter views

[ryelle]

Fixes #180 — This updates the query-title block so that it always outputs heading text on filtered archive views. The heading is still invisible, it's currently there for screen reader users & SEO.

For example:

• /archives/?tag[]=dining — "Filter by: Dining"
• /archives/?tag[]=dining&cat[]=3 — "Filter by: Dining, Featured"
• /archives/?cat[]=3&s=restaurant — Filter by: Featured, restaurant

To test

Try any number of filter combinations on the archives page. There should always be an h1 on the page, and the title should match the current filters. You can use a bookmarklet like this one to test.

Also check the document title, it should reflect the currently-selected filters too.

If you want, switch out the query-title in patterns/_site-grid.php with the following, for a visible heading.

<!-- wp:query-title {"type":"filter","style":{"spacing":{"margin":{"top":"-20px","bottom":"var:preset|spacing|40"}}}} /-->