[BUGFIX] Make admin modules only available for admins

The extension manager and backend user module
are targeted for admins only.
Reflect that in the module configuration.

Releases: 6.2, 6.1, 6.0
Change-Id: I3604fd245c712a41f70c6e1672a238dcdbaaeddc
Reviewed-on: https://review.typo3.org/25186
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer