Introduce conditional controller instances (v3)

[dnlkoch]

Backport to the main in #251.

This makes the creation of all controller instances configurable, e.g. via the following entries in the application.properties:

controller.applications.enabled=false
controller.files.enabled=true
controller.groups.enabled=false
controller.identities.enabled=false
controller.imagefiles.enabled=true
controller.layers.enabled=true
controller.roles.enabled=false
controller.users.enabled=true

Once accepted, this should also be applied to the master branch.

Please review @terrestris/devs.

[marcjansen]

Very nice