Twenty Fifteen: clean up of `twentyfifteen_color_scheme_css_template()` in customizer.php. Props iamtakashi. Fixes #30601.

git-svn-id: https://develop.svn.wordpress.org/trunk@30763 602fd350-edb4-49c9-b593-d223f7449a82
Admin toolbar: remove unneeded overrides of `z-index` and `left` on screens narrower than 782px. Props avryl, babbardel. Fixes #29867.

git-svn-id: https://develop.svn.wordpress.org/trunk@30764 602fd350-edb4-49c9-b593-d223f7449a82
WP_HTTP: Revert r30491 which updated the bundled root certificates. There's a report that this is breaking under certain PHP/OpenSSL versions (which we've encountered before), and we're safer with a slighty out of date CA bundle than breaking HTTPS communication on affected sites.
See #30434


git-svn-id: https://develop.svn.wordpress.org/trunk@30765 602fd350-edb4-49c9-b593-d223f7449a82
In the `links_add_base()` callback, use a HTTP-aware URL joining function, rather than a filesystem joining function to avoid PHP warnings on certain systems.
Fixes #29950


git-svn-id: https://develop.svn.wordpress.org/trunk@30766 602fd350-edb4-49c9-b593-d223f7449a82