Added the pg gem.

[#78038926]
Added active_record configuration.

[#78038926]
Added a model to represent metrics for a single visit.

[#78038926]
Added the database_cleaner gem.

[#78038926]
MetricsLogger now uses VisitMetricsEntry.

[#78038926]
Added a migrator.

[#78038926]
Removed no longer used pieces, changed templates, adjusted CalculatedMetrics.
Removed references to redis from the app, as we're no longer using it for caching.

[Finishes #78038926]
Attempt to fix a bug in logstasher.
Updated remote repository ref for logstasher.
Added missing weekly metrics.
Merge pull request #41 from ministryofjustice/postgres-78038926

This introduces Postgres as a backing store for metrics.