Introduce conditional controller instances

[dnlkoch]

Backport of #180 and #185 to the main branch.

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

controller:
  applications:
    enabled: true
  cache:
    enabled: true
  files:
    enabled: true
  groups:
    enabled: true
  imagefiles:
    enabled: true
  layers:
    enabled: true
  users:
    enabled: true

Please review @terrestris/devs.