Administration: Improve the plugin and theme auto-update emails.

This makes several improvements to the emails sent to site admins when plugin and theme auto-updates are attempted.

- Emails are now specifically tailored to the three results (success, failed, or mixed).
- Subject lines and body copy for mixed and failed results now correctly convey the importance of double checking the site in question.
- The site’s URL is now included in all emails.
- When failures occur, links to the Plugins and/or Themes pages in the admin are now included so that site owners can easily take action.

Props audrasjb, desrosj, azaozz, garrett-eclipse, paaljoachim, johnbillion, marybaum, pbiron.
Follow up to [47835].
See #50052.
Fixes #50268.
Built from https://develop.svn.wordpress.org/trunk@48123


git-svn-id: https://core.svn.wordpress.org/trunk@47892 1a063a9b-81f0-0310-95a4-ce76da25c4cd