Mongo driver upgrade

[Nydauron]

Closes #8. Ports the current database helper functions to use mongo/mongo-go-driver

Still need to:

• [x] Add timeouts to context (add ctx property to Database struct)
• Fix multi-document CRUDs from data races (implement ACID transactions)
• [x] Port e2e tests over to use new driver
• [x] Remove all references to mgo

[Nydauron]

This PR changes nothing to any of the endpoints. All the endpoints should behave identically prior to the driver upgrade.

Throughout implementing dtm into the API, endpoints will be changed to use transactions if need be.