Normalise broadcast content before validating length

This changes the content length validation of the internal API to match
the validation of the public broadcast API[1].

This removes the length check from JSONSchema, which isn’t sophisticated
enough to deal with things like normalising newlines or handling
different encodings.

The admin app should catch these errors before they’re raised here, but
it’s best to be belt and braces.

1.7ab0403ae7/app/v2/broadcast/post_broadcast.py (L53-L63)

tests/app/broadcast_message/test_rest.py

@@ -192,6 +192,40 @@ def test_create_broadcast_message_400s_if_json_schema_fails_validation(
     assert response['errors'] == expected_errors
 
 
+@pytest.mark.parametrize('content, expected_status, expected_errors', (
+    ('a', 201, None),
+    ('a' * 1_395, 201, None),
+    ('a\r\n' * 697, 201, None),  # 1,394 chars – new lines normalised to \n
+    ('a' * 1_396, 400, (
+        'Content must be 1,395 characters or fewer'
+    )),
+    ('ŵ' * 615, 201, None),
+    ('ŵ' * 616, 400, (
+        'Content must be 615 characters or fewer '
+        '(because it could not be GSM7 encoded)'
+    )),
+))
+def test_create_broadcast_message_400s_if_content_too_long(
+    admin_request,
+    sample_broadcast_service,
+    content,
+    expected_status,
+    expected_errors,
+):
+    response = admin_request.post(
+        'broadcast_message.create_broadcast_message',
+        service_id=sample_broadcast_service.id,
+        _data={
+            'service_id': str(sample_broadcast_service.id),
+            'created_by': str(sample_broadcast_service.created_by_id),
+            'reference': 'abc123',
+            'content': content,
+        },
+        _expected_status=expected_status,
+    )
+    assert response.get('message') == expected_errors
+
+
 @freeze_time('2020-01-01')
 def test_create_broadcast_message_can_be_created_from_content(admin_request, sample_broadcast_service):
     response = admin_request.post(