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`.

app/main/views/service_settings.py

@@ -29,6 +29,7 @@ from app import (
     user_api_client,
 )
 from app.extensions import zendesk_client
+from app.formatters import email_safe
 from app.main import main
 from app.main.forms import (
     BrandingOptions,
@@ -59,7 +60,6 @@ from app.utils import (
     DELIVERED_STATUSES,
     FAILURE_STATUSES,
     SENDING_STATUSES,
-    email_safe,
     user_has_permissions,
     user_is_gov_user,
     user_is_platform_admin,