Start of SMS game creation

#### What's this PR do?
Creates models and controllers that'll be need to create an SMS game. Nothing actually happens yet, but want to get this layout of stuff reviewed first.

#### Where should the reviewer start?
The `/game/create` endpoint got added to `app/router.js`. The router behavior logic is then passed on to the modules created in the `app/controllers` folder. `models/sgGameSchema.js` is the base schema for our SMS games. `sgCollaborativeStory`, `sgCompetitiveStory` and `sgMostLikelyTo` return models that extend that base schema. They're not final, but they're a best guess right now at what we'll need.

#### How should this be manually tested?
POST to `http://localhost:4711/game/create` with a `type` parameter = `collaborative-story`, `competitive-story`, or `most-likely-to`. You should get a 200 OK returned with a placeholder message.

#### What are the relevant tickets?
Closes #32 
Starts work on #38

#### Questions:
- Any opinions on file naming conventions?