Add a confirmation checkbox for live broadcasts

We want people to be really sure before sending a live broadcast, not
just clicking through the green buttons.

This commit adds a checkbox which explains exactly the consequences of
what they’re about to do, tailored to the channel they’re on, and the
area chosen by the person creating the alert.

app/main/forms.py

@@ -1,3 +1,4 @@
+import math
 import weakref
 from datetime import datetime, timedelta
 from itertools import chain
@@ -1288,6 +1289,50 @@ class NewBroadcastForm(StripWhitespaceForm):
         return self.content.data == 'template'
 
 
+class ConfirmBroadcastForm(StripWhitespaceForm):
+
+    def __init__(self, *args, service_is_live, channel, max_phones, **kwargs):
+        super().__init__(*args, **kwargs)
+
+        self.confirm.label.text = self.generate_label(channel, max_phones)
+
+        if service_is_live:
+            self.confirm.validators += (DataRequired(),)
+
+    confirm = GovukCheckboxField("Confirm")
+
+    @staticmethod
+    def generate_label(channel, max_phones):
+        if channel == 'test':
+            return (
+                'I understand this will alert anyone who has switched '
+                'on the test channel'
+            )
+        if channel == 'severe':
+            return (
+                f'I understand this will alert {ConfirmBroadcastForm.format_number_generic(max_phones)} '
+                'of people'
+            )
+        if channel == 'government':
+            return (
+                f'I understand this will alert {ConfirmBroadcastForm.format_number_generic(max_phones)} '
+                'of people, even if they’ve opted out'
+            )
+
+    @staticmethod
+    def format_number_generic(count):
+        for threshold, message in (
+            (1_000_000, 'millions'),
+            (100_000, 'hundreds of thousands'),
+            (10_000, 'tens of thousands'),
+            (1_000, 'thousands'),
+            (100, 'hundreds'),
+            (-math.inf, 'an unknown number')
+        ):
+            if count >= threshold:
+                return message
+
+
 class BaseTemplateForm(StripWhitespaceForm):
     name = GovukTextInputField(
         "Template name",

app/main/views/broadcast.py

@@ -14,6 +14,7 @@ from app.main.forms import (
     BroadcastAreaForm,
     BroadcastAreaFormWithSelectAll,
     BroadcastTemplateForm,
+    ConfirmBroadcastForm,
     NewBroadcastForm,
     SearchByNameForm,
 )
@@ -401,6 +402,11 @@ def view_broadcast(service_id, broadcast_message_id):
             back_link_endpoint,
             service_id=current_service.id,
         ),
+        form=ConfirmBroadcastForm(
+            service_is_live=current_service.live,
+            channel=current_service.broadcast_channel,
+            max_phones=broadcast_message.count_of_phones_likely,
+        ),
     )
 
 
@@ -414,6 +420,12 @@ def approve_broadcast_message(service_id, broadcast_message_id):
         service_id=current_service.id,
     )
 
+    form = ConfirmBroadcastForm(
+        service_is_live=current_service.live,
+        channel=current_service.broadcast_channel,
+        max_phones=broadcast_message.count_of_phones_likely,
+    )
+
     if broadcast_message.status != 'pending-approval':
         return redirect(url_for(
             '.view_current_broadcast',
@@ -421,14 +433,15 @@ def approve_broadcast_message(service_id, broadcast_message_id):
             broadcast_message_id=broadcast_message.id,
         ))
 
-    broadcast_message.approve_broadcast()
-
     if current_service.trial_mode:
+        broadcast_message.approve_broadcast()
         return redirect(url_for(
             '.broadcast_tour',
             service_id=current_service.id,
             step_index=6,
         ))
+    elif form.validate_on_submit():
+        broadcast_message.approve_broadcast()
 
     return redirect(url_for(
         '.view_current_broadcast',