Ability to collapse Site's Pages

[lordtct]

It would be great if not just page sections, but the entire site's pages be collapsible in the manager page list. This would significantly enhance the quality of life when working with multiple sites with large number of pages.

Example:

Adding collapse via Bootstrap would be trivial. I could do a pull request and hopefully it would be in time for release 9.0.

Referenced issues: #1260