Re-implement unique constraints on categories/syndication links

Although I disabled it in !122 (merged), for ease of implementation, I need to re-enable it, otherwise we'll have some annoying duplications of tags/URLs that don't need to be there.

We can do this using the Sanitisers, as they're run just before committing the file, so ensures that they're resolved before it reaches the repo.

Closes #186 (closed).