Autofill category and mechanic entities

[frowenz]

This issue should be addressed after #393.

Currently, the autofilling functionality for admins creating new games as well as the code to generate game fixtures does not parse category and mechanic information. The main difficulty is the Categories and Mechanics are their own entities in the database.

Steps:

1. Parse all the categories and mechanics information.
2. Associate each category with the corresponding primary key of each category or mechanic entity. This likely involes parsing the information in this fixture, src/chigame/games/fixtures/mechanics_categories_fixtures.json
3. Record this information appropriately. For Game Creation Autofilling, this means populating the game creation form. For Fixture Creation, this means creating a separate entity in the fixture json to record this relationship.