Twenty Fifteen: Don't save unfiltered CSS in a setting.

Introduce `twentyfifteen_get_color_scheme_css( $colors )` which will be used for Underscore/Backbone and the PHP side. Let `twentyfifteen_color_scheme_css()` handle the color calculation on PHP side.

Merges [30893] to the 4.1 branch.

Fixes #30409.
Props ocean90

Built from https://develop.svn.wordpress.org/branches/4.1@30927


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30916 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1 RC2

Built from https://develop.svn.wordpress.org/branches/4.1@30928


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30917 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Last about page string. Merges [30929] to the 4.1 branch.

props celloexpressions.
fixes #30435.

Built from https://develop.svn.wordpress.org/branches/4.1@30930


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30919 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Add `_wp_attachment_is_custom_background` meta to uploaded background images.

Merges [30885] to the 4.1 branch.

props ocean90.
fixes #30707.

Built from https://develop.svn.wordpress.org/branches/4.1@30931


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30920 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: Use dynamic default color value instead of hardcoded color.

Merges [30932] to the 4.1 branch.

props iamtakashi.
see #30733.

Built from https://develop.svn.wordpress.org/branches/4.1@30933


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30922 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix MEjs styles for screen reader text and overflow on the volume slider.

Merges [30907] to the 4.1 branch.

props obenland.
see #30281.

Built from https://develop.svn.wordpress.org/branches/4.1@30934


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30923 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Merge Twenty Fifteen commits [30828], [30833], [30869] to the 4.1 branch.
Built from https://develop.svn.wordpress.org/branches/4.1@30935


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30924 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Merge Twenty Fifteen commits [30834], [30845] to the 4.1 branch.
Built from https://develop.svn.wordpress.org/branches/4.1@30936


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30925 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen, editor-style: remove margin-bottom from the gallery preview inside the editor.

Merges [30836] to the 4.1 branch.

props azaozz.
see #30697.
Built from https://develop.svn.wordpress.org/branches/4.1@30937


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30926 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: update scroll functionality to make sure child menu items are always reachable.

Merges [30838] to the 4.1 branch.

props iamtakashi.
see #30674.
Built from https://develop.svn.wordpress.org/branches/4.1@30938


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30927 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: fallback hex color for no-comments text should come first.

Merges [30910] to the 4.1 branch.

props iamtakashi.
see #30730.
Built from https://develop.svn.wordpress.org/branches/4.1@30939


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30928 1a063a9b-81f0-0310-95a4-ce76da25c4cd