feature: Allows check_integrity command to get expended from packages

Changed the way check_integrity command works to allow any app package to add its own checks.

my_server_folder/
  djangoldp/
    check_integrity.py
  my_app/
    check_integrity.py
  manage.py
  ...

When running ./manage.py check_integrity, every check_integrity.py will ran, ordered by the DJANGOLDP_PACKAGES variable.