Implement STI on User model

[nicklathe]

This PR adds STI (Single Table Inheritance) to our User model. There are two new classes introduced, Teacher and Student that both inherit from the User base class. One benefit of implementing STI on the User model is that it allows us organize the model along the axis of user_type and move student and teacher specific logic into their respective classes.

This functionality should not break existing usage. For example, you can continue to create a new user by calling User.create(user_type: "Teacher"). You can also now instead call Teacher.create(), as STI provides additional class methods.

This PR adds the following changes:

• Creates teacher.rb and student.rb and associated classes
• Adds STI inheritance column override (default is type, this overrides that to be user_type
• Adds find_sti_class method override
• Adds logic to set the user class in user_builder.rb
• Updates unit tests to handle the introduction of the new STI classes

Links

Testing story

You can test locally, and will be able to create new teacher and student accounts as before, as well as downgrade/upgrade account styles (Teacher -> Student, Student -> Teacher.

Deployment strategy

Follow-up work

Privacy

Security

Caching

PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[nicklathe]

Rails prepends the user class for these attributes because the form builder knows the object is a User model. However since a user is now either a Teacher or Student class, these attributes are now being prepended with student_ rather than user_. This is probably obvious to those very familiar with Rails, but I was unaware of this prepending logic.