Bug 31401: Update administration sidebar to match entries on administration start...

Bug 31401: Update administration sidebar to match entries on administration start page

This makes sure that all entries on the administration start page
have a matching entry in the administration sidebar that shows
on the left side if you are on any of the administration sub pages.

Changes made:
* Rename 'Classification sources' to 'Classification configuration'
* Make Plugins entry show and appear in correct spot
  This relied on the variable plugins_enabled that wasn't available
  in all the different templates. I therefore moved it to Auth.pm
  and cleaned up the code for the admin start page.
* Move 'MARC overlay rules' and rename to 'Record overlay rules'

To test:
* Make sure plugins are enabled and visible on admin start page
* Compare admin start page and sidebar
  * Sequence should be the same
  * All entries should appear on both pages
  * Naming should be the same

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>