API Tech Debt

[timcowlishaw]

A rolling list of improvements and refactors to tackle as we go:

• [x] #246
• [ ] Remove all references to deprecated user avatar property.
• [ ] Improve test coverage on MQTTTask, TelnetTask, Storers and related classes, and refactor
• [ ] Improve test coverage on device_archive job and refactor.
• [ ] Refactor jbuilder templates to settle on a 'standard' representation of each model and remove duplication
• [ ] Refactor out deprecated sensor_map abstraction on device.

[oscgonfer]

https://github.com/fablabbcn/smartcitizen-api/issues/279