Backend enhancements

[jackxujh]

Title

Backend enhancements

Description

This PR introduces numerous improvements to the backend system and documentation:

• Add symmetricds references to project readme #560
• Save errors of received measurements #569
• Sync mercury_* tables #592
• Remove duplicated radiorecevier.py #602
• Refactor AGActiveEvent model to Singleton #598
• Record error when no active event #612
• Finalize Contributing.md file #632

Types of Changes

Put an x in the boxes that apply

• [x] Feature (non-breaking change which adds functionality)
• [x] Bug Fix (non-breaking change that fixes an issue)
• [ ] Breaking Change (feature/fix that causes existing features to not work as expected)
• [ ] Documentation

Checklist

• [ ] I have read the [contribute]contributing doc
• [ ] Classes, scripts, and environment variables follow existing naming convention
• [ ] Lint and Unit tests pass locally
• [ ] New features on hardware have been tested on a local Raspberry Pi
• [ ] Mention new programs/binaries if any must be installed along with this change
• [ ] Mention new environment variables if any have been added to hardware/env file
• [ ] Test coverage should not drop more than 3%
• [ ] Any new required python modules are added to the requirements.txt