Network Admin: Properly check if the theme is active on the main site to avoid deleting it via bulk actions.

props ocean90.
fixes #29861.
Built from https://develop.svn.wordpress.org/trunk@29839


git-svn-id: http://core.svn.wordpress.org/trunk@29603 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Translate some previously untranslated _doing_it_wrong() messages.

props georgestephanis.
fixes #25614.
Built from https://develop.svn.wordpress.org/trunk@29840


git-svn-id: http://core.svn.wordpress.org/trunk@29604 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add default z-index to the admin menu, see #29806
Built from https://develop.svn.wordpress.org/trunk@29841


git-svn-id: http://core.svn.wordpress.org/trunk@29605 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Bundled themes: Add an alt attribute with the site title for header images linked to the home page.

props rianrietveld.
fixes #15926.
Built from https://develop.svn.wordpress.org/trunk@29842


git-svn-id: http://core.svn.wordpress.org/trunk@29606 1a063a9b-81f0-0310-95a4-ce76da25c4cd