Allow all database models to be extended

[gunthercox]

A common request is that a developer wants to add an additional field to one of the database models. To easily allow this and also to encourage loose coupling of ChatterBot with its database models, changes should be made so that it is possible to specify the model classes that a chat bot should use.

• Django models
• SQL Alchemy models
• Python object models
• Remove the extra_data field in a future pull request because this is a way better method to facilitate adding custom fields.
• Blocks Document how to create and use custom database models #1097

[gunthercox]

The extra_data attribute has been removed as a part of #1402