Allow adding broadcast templates

At the moment the page is the same as for text message templates,
except:
- different H1
- no guidance about personalisation, links, etc (until we decide how
  these should work)

For now you won’t be able to really create a broadcast template, because
the API doesn’t support it (the API will respond with a 400). But that’s
OK because no real services have the broadcast permission yet.

This required a bit of refactoring of how we check which template types
a service can use, because there were some hard-coded assumptions about
emails and text messages.

app/main/forms.py

@@ -780,6 +780,10 @@ class SMSTemplateForm(BaseTemplateForm):
         OnlySMSCharacters()(None, field)
 
 
+class BroadcastTemplateForm(SMSTemplateForm):
+    pass
+
+
 class LetterAddressForm(StripWhitespaceForm):
 
     def __init__(self, *args, allow_international_letters=False, **kwargs):
@@ -1644,7 +1648,7 @@ class TemplateAndFoldersSelectionForm(Form):
         self,
         all_template_folders,
         template_list,
-        allow_adding_letter_template,
+        available_template_types,
         allow_adding_copy_of_template,
         *args,
         **kwargs
@@ -1652,6 +1656,8 @@ class TemplateAndFoldersSelectionForm(Form):
 
         super().__init__(*args, **kwargs)
 
+        self.available_template_types = available_template_types
+
         self.templates_and_folders.choices = template_list.as_id_and_name
 
         self.op = None
@@ -1664,12 +1670,25 @@ class TemplateAndFoldersSelectionForm(Form):
         ]
 
         self.add_template_by_template_type.choices = list(filter(None, [
+            # We want to show email and text message to everyone,
+            # whether or not the service has them switched on. The
+            # option to add letter or broadcast templates should only
+            # be shown to services which have that permission
             ('email', 'Email'),
             ('sms', 'Text message'),
-            ('letter', 'Letter') if allow_adding_letter_template else None,
+            ('letter', 'Letter') if 'letter' in available_template_types else None,
+            ('broadcast', 'Broadcast') if 'broadcast' in available_template_types else None,
             ('copy-existing', 'Copy an existing template') if allow_adding_copy_of_template else None,
         ]))
 
+    @property
+    def trying_to_add_unavailable_template_type(self):
+        return all((
+            self.is_add_template_op,
+            self.add_template_by_template_type.data,
+            self.add_template_by_template_type.data not in self.available_template_types,
+        ))
+
     def is_selected(self, template_folder_id):
         return template_folder_id in (self.templates_and_folders.data or [])