Remove checks for unknown rating/relationship tags for non-AO3 archives

Superlove (among others) has added custom ratings and relationships kinds to the default ones. We currently check they fall within the AO3 known types, but should probably stop doing that. It may, however, have implications on our types.

We could offer presets of these settings for different archives to help people take these differences into account.

@xxwhirlpool can provide a few example of fics with these special settings that we can use for testing.