generic datamodel for chillhub+devices

A way to represent a datamodel on Firebase.

Inspired by what found in Firebase Datasets: https://www.firebase.com/docs/open-data/transit.html#section-data-structure

This datamodel is supposed to be generic and should work for chillhub and any connected devices.

main changes:

* a fridge container will list in `objects`, the different devices connected to it.

Those devices could be containers or objects.
For example, In the case of a beerminder, it should be a container with an array of objects to beers.

* containers should have `compType` to defined which type of container they are

+1 @udryan10 - we need to be able to ascertain the path to a node definitively. Having IDs that could represent nodes under `/containers` and `/objects` in the same set of hash keys is problematic.