Toolbar: Display a "View Post" link when viewing comments for a single post on Comments screen.

Props Otto42, matt, SergeyBiryukov.
Fixes #50312.
Built from https://develop.svn.wordpress.org/trunk@47903


git-svn-id: https://core.svn.wordpress.org/trunk@47677 1a063a9b-81f0-0310-95a4-ce76da25c4cd