Allow advanced users to download a copy of the CatDat database

[ScriptRaccoon]

This PR offers advanced users to download a snapshot of the database and inspect the data in a terminal or with any database tool of their choice.

The download link is located in a new page /download. That page also lists a couple of interesting example queries.

How it looks like

Interesting Example Query

-- Top 10 properties with the most undecided categories
SELECT
	p.id AS property_id,
	COUNT(c.id) AS undecided_categories
FROM properties p
CROSS JOIN categories c
LEFT JOIN category_property_assignments cp
	ON cp.category_id = c.id
	AND cp.property_id = p.id
WHERE cp.property_id IS NULL
GROUP BY p.id
ORDER BY undecided_categories DESC LIMIT 10;

Separation of Concerns

This feature is only possible because previously (#39 + #119) a clear cut was made between the CatDat database (which contains mathematical knowledge) and the application database (user submissions + page visits + more in the future).