Custom Headers: Simplify and consolidate the querying of custom headers for the customizer.

props mcsf.
see #21785.

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3.9-beta3

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Post beta3 version bump.

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Restore some CSS rules that are necessary for non-default schemes. see #18380.

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Revert `mediaelement-and-player.min.js` to a usable build that doesn't cause an infinite loop. Still `2.14.0`, minus the offending pull request: https://github.com/johndyer/mediaelement/commit/a7f67b71800af1dd742162f1be3ffe401b72b1f9

Reported upstream. `2.14.0` is necessary for playing `m4a` files properly. `2.14.0` with the offending pull request is unusable in Chrome.

See #27582.


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Widget Customizer: Remove strings from WP_Error returns and go with just IDs.

fixes #27419.

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Adjust colors in wp_die() and install.css.

Missed in [26788] and [26837], see #25858.
fixes #27589.
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Customizer: Style "Cheatin’ uh?" message like wp_die().

props jackreichert.
fixes #27490
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Editor:
- Use standard button styles for the Quicktags buttons.
- Better style for the TinyMCE buttons (hover/focus/active/disabled).
- Move the fullscreen (DFW) button to the right.
- Better style for the Visual/Text buttons in DFW.
Props avryl, see #27279
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Add class `wp-core-ui` to DFW and wpLink as it is required by buttons.css. Props avryl, see #27279

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Only show test cookie warnings on submit as caching/proxies may intercept the test cookie for GET requests.

Introduce a new string for when headers are sent and link them to http://codex.wordpress.org/Cookies (new page).

props SergeyBiryukov.
fixes #27373.

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