tidy ProductFootprint model

[JohnVonNeumann]

The ProductFootprint model was the first model that was added, and it was done in a fairly POC friendly manner to just get it out and done, as a result, it is Not Good. It needs to be cleaned up.

• snake_case the field names
• rename it from ProductFootprint to ProductFootprintModel to keep the new standard, so it's easy to differentiate between the PFSchema and the soon to come ProductFootprint domain object for internal use and translation.
• massively overhaul the test suite, it just needs more
• go through the model itself and ensure that mandatory data fields are set to required=True, even the id field isnt currently mandatory

out of scope:
egregious levels of normalisation, if it makes sense and is quick, then do it, but it does not need to be entirely broken out yet.