Correctly validate Inside Gov slugs

Clashing Inside Government slugs can be differentiated by automatically appending `--$version` to their values. Prior to this commit this type of slug, which is valid and in public use, wouldn't pass validation.

We now special case that type of slug. As a bonus there are tests for the edge cases in the `SlugValidator` class.

This is needed to ensure we can actually push all of Inside Gov's content into Panopticon.