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/test_utils.py

@@ -10,9 +10,9 @@ from freezegun import freeze_time
 from notifications_utils.template import Template
 
 from app import format_datetime_relative
+from app.formatters import email_safe, round_to_significant_figures
 from app.utils import (
     Spreadsheet,
-    email_safe,
     generate_next_dict,
     generate_notifications_csv,
     generate_previous_dict,
@@ -23,7 +23,6 @@ from app.utils import (
     is_less_than_days_ago,
     merge_jsonlike,
     printing_today_or_tomorrow,
-    round_to_significant_figures,
 )
 from tests.conftest import fake_uuid