Switch Twitter oEmbed to SSL due to a Twitter API change. Ah, the open web.

props yurivictor, cojennin.
see #26844.

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Realign the oEmbed providers. see #26844.

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Add a contrasting border to admin feature pointers.

props iammattthomas.
see #26689.

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Unit tests for get_url_in_content(). Return false when no content is passed, to match the return value of no links being found.

props mdbitz.
#26171.

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Add dfn, mark, and samp to kses.

props kraftbj.
fixes #24950.

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make_clickable: When cleaning up accidental links within links, account for the tag being split by newlines.

props dd32.
fixes #19028.

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Use the_title_attribute() in feed_links_extra(), rather than get_the_title().

props c3mdigital.
fixes #15046.

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