Clean up the example data seeds into a single seed that calls a class in the library #2382
Description
Currently the example_data seed (db/seeds/example_data/) is split across multiple seeds, with dependencies and global variables spread between them.
Refactor into a single seed that calls a separate class to populate the data, possibly with multiple classes for each type. Some simple unit tests can then be created to provide some sanity checks that it is populating the database correctly.
Make it so it is easier to maintain and add new example data in the future.