Use encryption module from utils

Now that the encryption module has been moved from this app to utils, we
can remove it from here (along with its tests) and import it from utils
instead. This also renames the `encryption.py` file to `hashing.py`,
since it no longer contains the encryption class.

app/models.py

@@ -29,7 +29,7 @@ from notifications_utils.template import (
 )
 from notifications_utils.timezones import convert_bst_to_utc, convert_utc_to_bst
 
-from app.encryption import (
+from app.hashing import (
     hashpw,
     check_hash
 )