Themes screen: Various IE8 adjustments and fixes.

props morganestes, matveb.
fixes #26541.

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Improve keyboard accessibility for the admin menu when in responsive mode, props SergeyBiryukov, fixes #26639 for trunk.
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Themes screen: Add aria-describedby attributes to provide better information to screen readers.

props joedolson for initial patch.
fixes #26602 for trunk.

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Autosave: don't set "submit" buttons to disabled right before submitting the form. They are not sent with the form data, see #25272.

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Let `is_attachment()` accept an $attachment parameter, similar to `is_page()` and `is_single()`. Adds Unit Tests for all 3.

Props alex-ye for the initial patch.
Fixes #24257.


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Fix a bit of whitespace from [27016].


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Fix imperceptible whitespace cruff from [27017].


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