New Mythology: Initial implementation

[mattlindsey]

Allow users to create 'custom' mythologies.

Initially this can just insert new records into the mythologies table, but this might be a good opportunity to allow users (if they want) to create an id on the site(https://github.com/heartcombo/devise) and show both the seeded mythologies (Greek, Roman, etc.) and any that they have created also.
If we add the Devise gem and allow users to create ids, we should keep it simple initially and it shouldn't be required to create an id for simple stories.