Configure base Mapit URL in an initializer.

To be overridden on deploy.  Also pointed at our Mapit instances.
Update authority details when importing.
Skip local interaction rows with invalid SNAC
Make local authority importer continue if a row errors.
Fix SNAC validation to allow county-level SNACs
Add exception_notifications on data import errors.

Required bumping exception_notifier to get
background_exception_notification support.
Merge pull request #70 from alphagov/fix_local_directgov_importer

Fix some issues with local directgov importer