Improvements to the extensions UI and update notifications.

Overview

Following a post on MatterMost from @DaveD we discussed how valuable the extension he wrote about and posted here: https://civicrm.org/node/11234 is and I think it's worth working into core. @KarinG then raised a related and equally valuable point that extensions managed outside of CiviCRM e.g. with Git or just existing on the filesystem are marked Green like up-to-date extensions when actually we lack enough data to determine if these extensions are, in fact, up-to-date.

Therefore I would like to fold Dave's extension's functionality into CiviCRM core and make changes to the extensions UI to remove the green highlight and possibly at a notice to extensions that don't exist on CiviCRM.org to indicate that we cannot determine whether these are up-to-date and extension authors should check manually.

In the discussion I proposed the following:

Core:

• Checks for extension updates if extension is on c.o/extensions but only updates those approved for in-app (at the moment at least!)
• Displays a message on extensions it doesn't manage (that don't exist on c.o) to the effect of "We can't check for updates for this extension." - similar to how Drupal displays a notice on git-managed or custom extensions.

Contrib:

• Could provide for update checking for git-managed and possibly custom extensions.

I have time available to work on this last week of April.