Flip the uniqueness constraint on `Mapping`.

While the effect is the same either way (that no two `Mapping` objects
can have the same values for both `path` and `site_id`), it's clearer
this way around that the idea is to ensure no two mappings exist for the
same path in a site.
Merge pull request #455 from alphagov/reword-uniqueness-constraint

Flip the uniqueness constraint on Mapping