Security: Add user interface to auto-update themes and plugins.

Building on core update mechanisms, this adds the ability to enable automatic updates for themes and plugins to the WordPress admin. 

Fixes: #50052.
Props: afercia, afragen, audrasjb, azaozz, bookdude13, davidperonne, desrosj, gmays, gmays, javiercasares, karmatosed, knutsp, mapk, mukesh27, netweb, nicolaskulka, nielsdeblaauw, paaljoachim, passoniate, pbiron, pedromendonca, whodunitagency, whyisjake, wpamitkumar, and xkon.

Built from https://develop.svn.wordpress.org/trunk@47835


git-svn-id: https://core.svn.wordpress.org/trunk@47611 1a063a9b-81f0-0310-95a4-ce76da25c4cd