remove ab-test flags, add logic checking for query params in abtesting b...

#### What's this PR do?
After thinking a bit harder, we realized that we didn't need to have an additional `abtest` flag on our POST requests, as we thought so with PR #307. 

1. This PR removes the `abtest=true` flag checks in our `createGame()` function.
2. In the `betaJoinGame()`, this PR first checks for the `request.query.args` value before it checks for the `request.body.args`, since the `request.body.args` value will always be reassigned to the keyword (in this case, "ACCEPT") in [this MData](https://secure.mcommons.com/mdatas/11789). 

#### How should this be manually tested?
Ran through a normal bully text game via Postman, and `npm test`. Will also test upon deploy. 

#### What are the relevant tickets?
Works on #306. 
Feels a bit weird with `query.args` basically acting as an override of `body.args` just because it's sorta unclear that that's what's happening on the request side of things. But I think it's ok for now since this `betaJoinGame` is a pretty specific use-case.