Basic story progression

#### What's this PR do?
The main thing done here is that `SGCompetitiveStoryController.userAction` is implemented to handle user responses to story prompts. If a valid answer is given, then the app saves the user's answer and progresses them onto the next item in the story.

Other minor thing done:
* 202 responses were changed to 200 because Mobile Commons sends error emails when they're anything but 200.

#### Where should the reviewer start?
`competitive-stories.json`
This file includes a test story defining what answers are valid for a given opt in path. And if a valid answer is given, then it defines the next opt in path that user should be sent to.

`SGCompetitiveStoryController.js`
`execUserAction` is a callback that gets called after a user's game is found. It uses the game document to find out what opt in path the user is currently at. Based on their answer (request.body.args) and the valid answers defined in the story config, it decides how to progress the story forward or to resend the last message to the user.

If the user is moved forward, it'll also save their previous answer and the current status in the db.

#### What are the relevant tickets?
Closes #69