In `wp.media.mixin`, add `ua` and a `compat` map to separate and clarify browser detection and support. Leverage Underscore list iterators in `isCompatible()`.

See #27389.



git-svn-id: https://develop.svn.wordpress.org/trunk@27542 602fd350-edb4-49c9-b593-d223f7449a82
Widget Customizer: Make temp hooks final and add inline docs.

New hooks are `dynamic_sidebar_before`, `dynamic_sidebar_after`, `dynamic_sidebar_has_widgets ` and `is_active_sidebar`.
Remove obsolete use of hacky dynamic_sidebar hook.

props westonruter, DrewAPicture.
fixes #25368.

git-svn-id: https://develop.svn.wordpress.org/trunk@27543 602fd350-edb4-49c9-b593-d223f7449a82
Merge branch 'master' into widget-customizer-patches