Added postgres.E005 to validate 'django.contrib.postgres' is in INSTALLED_APPS
when using:
* PostgreSQL-specific fields (ArrayField, HStoreField, range fields, SearchVectorField),
* PostgreSQL indexes (PostgresIndex and all subclasses), and
* ExclusionConstraint
The check provides immediate feedback during system checks rather than failing
later with obscure runtime and database errors.
Thanks to Simon Charette and Sarah Boyce for reviews.