Network Admin toolbar menu: Add Plugins and Themes, remove Visit Network.

props Ipstenu, SergeyBiryukov.
fixes #21674.


Built from https://develop.svn.wordpress.org/trunk@25120


git-svn-id: http://core.svn.wordpress.org/trunk@25100 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Avoid an undefined index notice when submitting a comment with JavaScript disabled. fixes #24752.
Built from https://develop.svn.wordpress.org/trunk@25121


git-svn-id: http://core.svn.wordpress.org/trunk@25101 1a063a9b-81f0-0310-95a4-ce76da25c4cd