Move Service Desk functionality into a separate module (file).

In preparation for much more functionality being added to this plugin.
Override WordPress' revisions limit to 5 per post/page.

Mostly needed because WP Engine disabled revisions by default
Description
Exclude /admin/ pages from WordPress search results
Is plugin active function
Purge cache when Custom CSS / header/footer code are saved.

Relies on these two new hooks:

https://github.com/OM4/om4-custom-css/commit/8bc33e10cb6d1ddc31327d9e0dd2a88c4d77d5bf
https://github.com/OM4/om4-header-footer-code/commit/2c6a161a3bbe1ce1d66f3446f68508560879b2e6
Revisions documentation.
Better detection of WP Engine's environment.
Add readme.
Clear cache when members only plugin settings are changed

Fixes Bugz 2622
Only include OM4_Plugin_Base class definition if necessary
Merge branch 'refs/heads/develop' into release