Add TinyMCE placeholders for `audio` and `video` shortcodes.

* Add `wp.media.mixin`.
* Add `wp.media.audio` and `wp.media.video`.
* Add `wp.media.model.PostAudio` and `wp.media.model.PostVideo`
* Add `wp.media.controller.AudioDetails` and `wp.media.controller.VideoDetails`.
* Add `wp.media.controller.ReplaceAudio` and `wp.media.controller.ReplaceVideo`.
* Add `wp.media.view.MediaFrame.AudioDetails` and `wp.media.view.MediaFrame.VideoDetails`.
* Add `wp.media.view.AudioDetails` and `wp.media.view.VideoDetails`.
* Update the `wpgallery` TinyMCE plugin.
* Display audio and video players in the media modal when shortcode is clicked.
* Provide a UI to edit shortcode attributes in the media modal.
* Provide a UI to replace the `src` media file in an `audio` or `video` shortcode.

See #27016.


Built from https://develop.svn.wordpress.org/trunk@27411


git-svn-id: http://core.svn.wordpress.org/trunk@27258 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add context and a missing period to widget descriptions. Fixes #26668. Props pavelevap, Hanni
Built from https://develop.svn.wordpress.org/trunk@27412


git-svn-id: http://core.svn.wordpress.org/trunk@27259 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix bulk activation of network-only plugins. Fixes #26487. Props dd32.
Built from https://develop.svn.wordpress.org/trunk@27413


git-svn-id: http://core.svn.wordpress.org/trunk@27260 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Audio doesn't have the same quirks as video in the media modal. We don't have to aggressively destroy the `mejs` instance.

See #27016.


Built from https://develop.svn.wordpress.org/trunk@27414


git-svn-id: http://core.svn.wordpress.org/trunk@27261 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add some missing JSDoc blocks to media-related code.

See #27016.


Built from https://develop.svn.wordpress.org/trunk@27415


git-svn-id: http://core.svn.wordpress.org/trunk@27262 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Indicate that the user deletion process affects all content attributed to a given user, not just posts. props seanchayes. fixes #26709.

Built from https://develop.svn.wordpress.org/trunk@27416


git-svn-id: http://core.svn.wordpress.org/trunk@27263 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove trailing whitespace in dashicons.css.
Built from https://develop.svn.wordpress.org/trunk@27417


git-svn-id: http://core.svn.wordpress.org/trunk@27264 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Introduce a `.dashicons-before` CSS class.

If you want to use a Dashicon before an element you can use the class because it will not change the elements content styling. With that you don't need to copy the entire `.dashicons` styling to your :before styling anymore.
To demonstrate this change, Admin Menu now uses Dashicons classes directly.

And it fixes a glitch in IE.

fixes #26630.
Built from https://develop.svn.wordpress.org/trunk@27418


git-svn-id: http://core.svn.wordpress.org/trunk@27265 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add widget management to the customizer.

This brings in the Widget Customizer plugin: https://wordpress.org/plugins/widget-customizer/.

props westonruter, shaunandrews, michael-arestad, johnregan3, akeda, topher1kenobe, topquarky, bobbravo2, ricardocorreia. And for good measure, props westonruter.
see #27112.

Built from https://develop.svn.wordpress.org/trunk@27419


git-svn-id: http://core.svn.wordpress.org/trunk@27266 1a063a9b-81f0-0310-95a4-ce76da25c4cd