Introduce 'rss_tag_pre' action, which fires between the xml and rss tags in a feed. 

props solarissmoke, wonderboymusic, DrewAPicture.
fixes #19017.
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Inline docs syntax fixes following [28978].

See #28740.

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Correct the usage of the variable passed to `wp_install_download_language_pack()` and kill an unused local variable. Props michalzuber. Also avoid using the `$_REQUEST` superglobal in this function. See #28577.
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Remove some unnecessary empty space in the media library grid view. props adamsilverstein. see #24716.

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Language on install: Docs and naming cleanups. see #28577.

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TinyMCE wpView:
- Add loadingPlaceholder() that returns some html to use as placeholder while the view is loading. Includes a subtle CSS based loading animation.
- Fix setContent(), it should empty the element before appending the new node.
- Change getHtml() to always return a string.
Fixes #28761.
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Focus and blur styling (via JS) for the install screen language selector. Props jorbin. See #28577
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Narrower layout and loading spinner for the install screen language selector. See #28577
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TinyMCE wpView:
- Improve the fake caret hide/show.
- Improve getView() speed.
- Add callback for showing the proper path when a view is selected. This currently doesn't work, will start working after we update TinyMCE.
props avryl, see #28567, #28595.
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Pre-commit autoprefixer fixes for media-views.css
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