Remove more references to MP6 and standardize on `.wp-ui-*` for prefixed generic class names. props nacin, kovshenin, fixes #26263.

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Twenty Fourteen: sanitize the values of Featured Content layout option, props kwight. Fixes #26408.
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Remove unneeded transition styles on adminmenu links that make the transition between expanded and collapsed menubars choppy in Safari and Chrome. Fixes #26406.
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Allow HTML in the theme name for the themes.php template.

props matveb.
fixes #26411.

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Twenty Fourteen: fix editor style discrepancies for `mark` and `ins`. Props sixhours, fixes #26413.
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Improve the description of the Network Admin Email option to better describe what the address will be used for. Fixes #23569 props nacin.

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westi forgot my full stop. see #23569.

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