Add backend endpoint for loction deletion. If location has links then raise an exception.

Add delete to the location frontend toolbar, controller and routes.

If an error is displayed in the linker form (when creating a new record), focus the form on this error.  It was easy to miss.  Also show the error messages for a location on the form (rather than the top level new/edit templates).

Refactor the JSONModelI18nMixin to allow for a single message to be I18n'ed on demand
Explicitly check if a location has any relationship models in the object graph before deleting.

Fix exception when indexing locations that have a value for 'classification'.  The resolver expected the resolving attribute values to be hashes.

When indexing an Accession, index whether they have any locations too.  Add a migration to force index these records.
Merge pull request #700 from hudmol/delete-me-a-location

[Delivers #65826472] Delete locations