Add context for 'Previous' and 'Next' theme strings.

see #28120, for trunk.
Built from https://develop.svn.wordpress.org/trunk@28286


git-svn-id: http://core.svn.wordpress.org/trunk@28114 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Use a consistent context to remove duplicate strings.

props pavelevap.
fixes #28123.
Built from https://develop.svn.wordpress.org/trunk@28287


git-svn-id: http://core.svn.wordpress.org/trunk@28115 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Translate a string added in 3.7 but never translated previously.

props andrezrv.
fixes #28025.
Built from https://develop.svn.wordpress.org/trunk@28288


git-svn-id: http://core.svn.wordpress.org/trunk@28116 1a063a9b-81f0-0310-95a4-ce76da25c4cd