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/forms.py

@@ -38,7 +38,7 @@ from wtforms import (
 from wtforms.fields.html5 import EmailField, SearchField, TelField
 from wtforms.validators import URL, DataRequired, Length, Optional, Regexp
 
-from app import format_thousands
+from app.formatters import format_thousands, guess_name_from_email_address
 from app.main.validators import (
     BroadcastLength,
     CommonlyUsedPassword,
@@ -60,7 +60,7 @@ from app.models.roles_and_permissions import (
     permissions,
     roles,
 )
-from app.utils import guess_name_from_email_address, merge_jsonlike
+from app.utils import merge_jsonlike
 
 
 def get_time_value_and_label(future_time):