Winston logging

#### What's this PR do?
This adds the winston logger to the app and uses it to log existing errors and the logs from the mobilecommons library.

You'll also find some separate cleanup work with the bitly and mobilecommons requires.

#### Where should the reviewer start?
`logger.js` creates an instance of the winston logger. 5 levels are available to be used, but by default we'll only be logging info and up on production. Logs will write to both the console as it did before, but now also to a MongoDB collection. Normal logs will go into the `logs_winston` collection.

Winston also automatically captures and logs unhandled exceptions. These will log to both the console and to a `logs_exceptions` collection in the MongoDB.

#### Any background context?
This is part of the ongoing effort to increase visibility of the status of the app in production and to more easily diagnose problems when they do arrive.

After pushing to production and seeing that this doesn't break any existing functionality, we'll gradually start migrating all `console.log` calls to use logger.

#### What are the relevant tickets?
#215 
Thanks for including this!