Login and Registration: Link to the correct site after activating a user on a...

Login and Registration: Link to the correct site after activating a user on a Multisite install in subdirectory mode.

Props pkarjala, Mista-Flo, tmoore41, Ipstenu, ksoares, mukesh27, whyisjake.
Fixes #39311.
Built from https://develop.svn.wordpress.org/trunk@48672


git-svn-id: https://core.svn.wordpress.org/trunk@48434 1a063a9b-81f0-0310-95a4-ce76da25c4cd