added retention metric

- included day to day active companies and average companies
- average active companies is calculated as the moving average from the past 60 days
- values in chart are to 5 decimal places
- added round to X decimal places to query helper service
- small login error message fix to show if the backend is not available

relates to https://github.com/Rise-Vision/Dashboard-proxy/pull/40

pr is hosted at http://dashboard.risevision.com:8000