Extract mongo integration code from repository

@gtrogers
@maxfliri
Change order of arguments to find

@robyoung
@maxfliri
Tests for mongo driver

@maxfliri
Inject mongo into repository

@robyoung
@maxfliri
Migrate part of repository test to unit tests

@robyoung
@maxfliri
Merge pull request #42 from alphagov/refactor-mongo-layer

Extract mongo integration code from repository