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.

tests/app/main/views/test_letters.py

@@ -14,7 +14,8 @@ letters_urls = [
     ([], 403)
 ])
 def test_letters_access_restricted(
-    platform_admin_client,
+    client_request,
+    platform_admin_user,
     mocker,
     permissions,
     response_code,
@@ -23,12 +24,12 @@ def test_letters_access_restricted(
     service_one,
 ):
     service_one['permissions'] = permissions
-
-    mocker.patch('app.service_api_client.get_service', return_value={"data": service_one})
-
-    response = platform_admin_client.get(url(service_id=service_one['id']))
-
-    assert response.status_code == response_code
+    client_request.login(platform_admin_user)
+    client_request.get_url(
+        url(service_id=service_one['id']),
+        _follow_redirects=True,
+        _expected_status=response_code,
+    )
 
 
 @pytest.mark.parametrize('url', letters_urls)