Unification of categories and functors

[ScriptRaccoon]

This PR is the frontend counterpart of #191. It is a major refactor of the whole frontend.

All functions, types, Svelte components, and Svelte pages that were previously defined separately for categories and functors (often with slight differences) have been unified. This eliminates a significant amount of code duplication, ensures that both categorical structures are handled consistently, and provides a foundation for supporting further categorical structures (#4) in the future.

In the process of aligning the pages, some features have been added to functors that were previously only available for categories:

• related functors, functor notation, functor tags, functor comments, related functor properties (this required schema changes)
• the consistency check on the functor search page, including display of contradictions (cf. Display contradiction proofs on the search results page #170)
• filtering on the functor list pages (functors, properties, implications)
• comparison page for functors

The unification is not finished yet: most database queries have not been unified. And those that have been unified (for example, for the search feature and the deduction scripts) are somewhat brittle, as they use string injection to construct a query that depends on the structure type (typical example: SELECT id FROM ${type}_properties). This will be improved in another PR.