Various refactorings

[ScriptRaccoon]

This PR contains various refactorings, most of which were already indicated by TODO comments in the code.

1. Breaking: The format of the YAML files has been simplified: related_categories → related, related_properties → related, dual_category → dual, and dual_property → dual. This reduces the amount of typing required and also allowed the seed types to be improved. Because of this change, this PR has touched hundreds of YAML files, but only in a trivial way.
2. Fields such as nlab_link and dual_property are now required, but may be null. (It was inconsistent before if they can be missing or null.) This makes missing data more visible and encourages it to be filled in. The same applies to left_adjoint (although it must of course be null for some functors). Overall, the files are now more consistent, and the seed types have been improved accordingly.
3. The negations of relations were originally stored in the database, but in a previous PR I had moved them to the frontend because they were only used there. This turned out to be a bad decision (increased indirection and duplication), so I reverted it. It was also more or less required for the next point.
4. The grammatical handling of preservation properties for functors, such as "preserves finite products", has been improved. Several approaches were tried, and while the current solution is still not ideal, it is a clear improvement over the previous workaround. In short, the empty relation '' is no longer allowed. Instead, the relation "preserves" is used for preservation properties. In property lists, this relation (or its negation, depending on the context) is displayed, while the "preserves " prefix is stripped from the property name.
5. I removed the implication terminal object => filtered, which has been obsolete since Add property: ℵ₁-cofiltered limits #200. It was only needed briefly while aleph1-filtered existed but aleph1-cofiltered did not, causing dualization to fail.