Move CellKey related code into more general models module.

This moves it right next to the cell table definition.
Check cell blacklist before creating new cell records, closes #175.

The blacklist only prevents the insertion of cell records, we still keep
the observations, just like we do for WiFi observations.
Merge pull request #178 from hannosch/175-respect-cell-blacklist

Respect cell blacklist and prevent re-creation of cell records.