Create a Procedure model and an example Page model

Page only contains a foreign key to a procedure for now.

Using this as a working branch and doing this in multiple pull requests.
Let's leave this as the default of `False`:

https://docs.djangoproject.com/en/dev/ref/models/fields/#null

>Avoid using null on string-based fields such as CharField and TextField because empty string values will always be stored as empty strings, not as NULL. If a string-based field has null=True, that means it has two possible values for “no data”: NULL, and the empty string. In most cases, it’s redundant to have two possible values for “no data;” the Django convention is to use the empty string, not NULL.