Twenty Fifteen: fix styles for screen-reader-only search button in small screens.

Props iamtakashi, fixes #30744.
Built from https://develop.svn.wordpress.org/trunk@30953


git-svn-id: http://core.svn.wordpress.org/trunk@30941 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Update individual term caches in `get_terms()`.

This was removed in [29915] as part of the attempt to add cache support to
`get_term_by()`. When that support was removed in [30900], it was not properly
restored.

This changeset includes a unit test to verify that the cache is properly primed
for terms found in `get_terms()`, as well as tests to verify the other cache
setting that was touched by [30900].

Fixes #30749. See #21760.
Built from https://develop.svn.wordpress.org/trunk@30954


git-svn-id: http://core.svn.wordpress.org/trunk@30942 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: ensure menu icon is properly centered for RTL views on small screens.

Props iamtakashi, fixes #30739.
Built from https://develop.svn.wordpress.org/trunk@30955


git-svn-id: http://core.svn.wordpress.org/trunk@30943 1a063a9b-81f0-0310-95a4-ce76da25c4cd