Make message and recipient counters formatters

As formatters we can use them in Jinja or Python code.

It also means we don’t need to import them every time we want to use
them – they’re always available in the template context.

For now this doesn’t remove the macros, it just aliases them to the
formatters. This gives us confidence that the formatters are working the
same way the old macros did, and reduces the diff size of each commit.

app/__init__.py

@@ -62,8 +62,14 @@ from app.formatters import (
     format_thousands,
     format_time,
     id_safe,
+    iteration_count,
     linkable_name,
+    message_count,
+    message_count_label,
+    message_count_noun,
     nl2br,
+    recipient_count,
+    recipient_count_label,
     valid_phone_number,
 )
 from app.models.organisation import Organisation
@@ -548,6 +554,12 @@ def add_template_filters(application):
         id_safe,
         convert_to_boolean,
         format_list_items,
+        iteration_count,
+        recipient_count,
+        recipient_count_label,
+        message_count_label,
+        message_count,
+        message_count_noun,
     ]:
         application.add_template_filter(fn)