Form validation in the Admin tool, for the Element fields

In the `README`, it says that the Element text must start with `start`, for Telegram to parse things correctly. I would recommend actually enforcing this in the form, rather than rely on human authors / process. You can do this with a custom form validation. See references below:

https://docs.djangoproject.com/en/1.11/ref/contrib/admin/#admin-custom-validation
https://docs.djangoproject.com/en/1.11/ref/forms/validation/
https://docs.djangoproject.com/en/dev/ref/validators/#writing-validators
https://stackoverflow.com/questions/12608639/django-field-validation-in-model-and-in-admin
https://stackoverflow.com/questions/24802244/custom-validation-in-django-admin

So something like `value.startswith('start')` would work?

And writing a test for that would be great...to verify the validator works as expected.

Start with the README requirement that Element text begins with `start`, then follow the Django admin and form-validation references to locate the Element form or validator. Add a validator and a test covering valid and invalid values; done means the admin rejects values without the required prefix.

Written by the indexing model from the issue text.