Indexes on Mongo collections

#### What's this PR do?
Indexes are set on models to improve query performance.

Also, did some minor cleanup in `mobilecommons.optout` where we no longer allow Mobile Commons credentials to be passed in through the function parameters.

#### Some notes on these changes
Mongoose documentation suggests that `autoIndex` be set to false since Mongoose will automatically call `ensureIndex()` for each defined index. Setting it to false disables this behavior and so avoids the performance costs of it. I've already separately created the indexes on all these collections in our production db and so far everything looks good.

#### How should this be manually tested?
I ran a couple local tests with Postman to ensure games could still be created and run through. Also `npm test` to ensure those tests didn't break.

#### What are the relevant tickets?
SMS-55, GH #25