Add a 6th (!) attribute to `wp_get_attachment_link()` to allow `aria-describedby` to be added to gallery output.

Props joedolson, DrewAPicture, rianrietveld.
Fixes #27402.

Built from https://develop.svn.wordpress.org/trunk@29914


git-svn-id: http://core.svn.wordpress.org/trunk@29667 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Cache `get_term_by()` calls:

* Add a helper function, `wp_get_last_changed()`, to retrieve a last-modified timestamp by cache group
* When caching a term, also make cache entries for slug and name via `slug:{$term_id}` and `name:{$term_id}` keys in the `$taxonomy:$last_changed` bucket that reference the term_id
* In `clean_term_cache()` and `update_term_cache()`, respect `$_wp_suspend_cache_invalidation`
* Original term cache entries maintain BC

Adds unit tests.

Props wonderboymusic, tollmanz, boonebgorges.
Fixes #21760.

Built from https://develop.svn.wordpress.org/trunk@29915


git-svn-id: http://core.svn.wordpress.org/trunk@29668 1a063a9b-81f0-0310-95a4-ce76da25c4cd