[Tests] Add model specs for Curriculum and CurriculumStory

[slaloggia]

New models need tests!
Optional: add factories for each

[mirandawang]

@rgenchev mentioned wanting to write tests for CurriculumStory model, so I'm assigning this ticket here :)

[rgenchev]

Thanks, @mirandawang! I'm on it! :)

[rgenchev]

@mirandawang, I decided to split this issue into two in order to have single-purpose PRs. Here are the two new issues: #547 and #548 . I think that this one can be closed.