Twenty Fifteen: minor corrections to borders and post title colors in custom color schemes.

Props iamtakashi, fixes #30521.



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Twenty Fifteen: overflow hidden can hide focus outlines here so out it goes.

Props iamtakashi, fixes #30523.


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Twenty Fifteen: better highlights for tapped links in iOS Safari.

Props iamtakashi, fixes #30524.



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Better error message when the admin is ssl and non-ssl oEmbed previews are not available in the editor. Props Japh, fixes #30533.
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Revert [30032] so appropriate PHP warnings aren't masked.

Fixes #28743

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Editor: use the Alt+Shift+W keyboard shortcut for new DFW from both the Visual and Text editors. Props afercia, fixes #30450.
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