Adds the /game/user-action endpoint + refactors how a user's game doc is found

#### What's this PR do?
`app/router.js`
Adds the `/game/user-action` endpoint. This is handled by `SGCompetitveStoryController.userAction()` which for now does nothing really.

`app/controllers/SGCompetitiveStoryController.js`
I refactored how `betaJoinGame` and `alphaStartGame` work. The process of having to find a user document, find the game mapping document, and then find the game document has been consolidated into the `findUserGame()` function.

`execBetaJoinGame` and `execAlphaStartGame` are now defined within `betaJoinGame` and `alphaStartGame` instead of as prototype methods. This should make the code a little more organized and easier to follow.