The function `import_exposure()` needs a keyword argument to define the number of items per batch during the import

The internal function _import_exposure() provides a keyword argument for the number of tiles per query during the import process. The public function import_from_postgis() uses this internal function but does not provide the keyword argument, thus rendering it impossible to change the batch size. This argument needs to be added.

\rfc @pdlmora @laurens @tara