Audio/Video shortcodes in the media modal:

* Make a generic model, `wp.media.model.PostMedia`, which replaces `wp.media.model.PostAudio` and `wp.media.model.PostVideo`
* Make a generic library, `wp.media.controller.MediaLibrary`, which replaces `wp.media.controller.ReplaceVideo` and `wp.media.controller.ReplaceAudio`
* `wp.media.controller.MediaLibrary` is used to create new states that want to load a library filtered by type, making it incredibly simple to add states to frames. See `wp.media.view.MediaFrame.VideoDetails` and `wp.media.view.MediaFrame.AudioDetails`.

UX changes:

* Add the ability to manage HTML5 fallbacks - add additional `<source>`s or remove specific `<source>`s
* In the Video Details state, add the ability to select a poster image

See #27016.


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git-svn-id: http://core.svn.wordpress.org/trunk@27322 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add `wp.media.view.MediaFrame.MediaDetails`, which `wp.media.view.MediaFrame.AudioDetails` and `media.view.MediaFrame.VideoDetails` extend. The subclasses subsequently only need to set `createStates()` and `bindHandlers()`, as well as any missing toolbar views.

See #27016.


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git-svn-id: http://core.svn.wordpress.org/trunk@27323 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Since audio and video shortcodes don't point at actual attachments, don't persist the library's selection. Move the media instructions in the block that scrolls.

See #27016.


Built from https://develop.svn.wordpress.org/trunk@27480


git-svn-id: http://core.svn.wordpress.org/trunk@27324 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Video editing in the media modal:

* Add a state: `Add Subititles`
* Add `text/vtt` to the list of allowed mime-types, files end in `.vtt`. `.srt` files are served as `text/plain`.
* The content body of a video shortcode should be used for adding `<track>` elements only. This happens dynamically in the modal. If added by hand, they can still be parsed and managed.

See #27016.


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git-svn-id: http://core.svn.wordpress.org/trunk@27325 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Admin Menu: Set default value of `$icon_url` in `add_menu_page()` to `dashicons-admin-generic`, fixes #26630.
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git-svn-id: http://core.svn.wordpress.org/trunk@27326 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Editor: Show 'View Post' for any post the author can read.

This expands it to private posts and matches the logic in the toolbar.

props mordauk, SergeyBiryukov.
fixes #27059.

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git-svn-id: http://core.svn.wordpress.org/trunk@27327 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Allow for custom authentication handlers for all requests.

Turn the logic used by wp_get_current_user() into a determine_current_user filter.

props rmccue.
fixes #26706.

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