Extract formatters into their own module

We have lots of functions for converting various types of data into
strings to be displayed to the user somewhere.

This commit collects all these functions into their own module, rather
than having them cluttering up `app/__init__.py` or buried amongst
various other things that have ended up in `app/utils.py`.

tests/app/main/views/test_service_settings.py

@@ -12,7 +12,7 @@ from notifications_python_client.errors import HTTPError
 from notifications_utils.clients.zendesk.zendesk_client import ZendeskClient
 
 import app
-from app.utils import email_safe
+from app.formatters import email_safe
 from tests import (
     find_element_by_tag_and_partial_text,
     invite_json,