Extract mongo integration code from repository

@gtrogers
@maxfliri
Refactor logging and change log file name

- Move duplicate setup into log_handler module
- Use [environment].log for both read and write
  When deployed read and write are their own applications so there is no
  overlap.
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
Merge pull request #41 from alphagov/logging-refactor

Refactor logging and change log file name