Add Two Tables and Update One

## Description
Adds a note_snippets table to allow a many to many relationship between notes and users. The type of the snippets will be different for each user so the type is stored in the note_snippets table as well.

Also adds a valid_schools table which will allow users to access classes in only the schools that the user has confirmed via email.

Finally the table teacher_classes was renamed to note_classes since that name makes more sense. 

### Testing Considerations
Really none. This will break the notebook page as it stands right now. Since this is being replaced soon... I'm not going to worry about it.