Nillify location on postcode update.

Allow for geocoding updates where a postcode changes on a Place by setting the location attribute to nil.
Perform geocoding before update
Assert that geocoding is not performed on data set import

The method handle_postcode_change should only perform geocoding when a postcode
is updated. It would be expensive to do this when importing a data set, these
additional assertions infer that geocoding has not been performed or attempted
when importing a new data set.
Merge remote-tracking branch 'origin/fix-place-update-geocoding'