Add filter for formatting a number as currency

This is used on the usage page, but is likely to become useful in other
places now that letter rates can be greater than £1.

app/__init__.py

@@ -521,6 +521,13 @@ def nl2br(value):
     return ''
 
 
+def format_number_in_pounds_as_currency(number):
+    if number >= 1:
+        return f"£{number:,.2f}"
+
+    return f"{number * 100:.0f}p"
+
+
 @login_manager.user_loader
 def load_user(user_id):
     return User.from_id(user_id)
@@ -798,6 +805,7 @@ def add_template_filters(application):
         format_notification_status_as_time,
         format_notification_status_as_field_status,
         format_notification_status_as_url,
+        format_number_in_pounds_as_currency,
         formatters.formatted_list,
         nl2br,
         format_phone_number_human_readable,

app/main/views/dashboard.py

@@ -456,7 +456,7 @@ def format_letter_details_for_month(letter_units_for_month):
 
         letter_details = LetterDetails(
             billing_units=sum(x['billing_units'] for x in rate_group),
-            rate=format_letter_rate(rate_group[0]['rate']),
+            rate=rate_group[0]['rate'],
             cost=(sum(x['billing_units'] for x in rate_group) * rate_group[0]['rate']),
             postage_description=rate_group[0]['postage']
         )
@@ -472,13 +472,6 @@ def get_postage_description(postage):
     return 'international'
 
 
-def format_letter_rate(number):
-    if number >= 1:
-        return f"£{number:,.2f}"
-
-    return f"{number * 100:.0f}p"
-
-
 def get_free_paid_breakdown_for_month(
     free_sms_fragment_limit,
     cumulative,

app/templates/views/usage.html

@@ -110,7 +110,7 @@
               {% for letter in month.letters%}
                 {% if letter.billing_units %}
                   <li class="tabular-numbers">{{ "{:,} {}".format(letter.billing_units, letter.postage_description) }} {{ message_count_label(letter.billing_units, 'letter', '') }}at
-                  {{ letter.rate }}</li>
+                  {{ letter.rate | format_number_in_pounds_as_currency }}</li>
                 {% endif %}
               {% endfor %}
               {% if not (month.free or month.paid or month.letters) %}