Move some DB queries to `db` package

[modrzew]

Keeping things separate - there should be no raw SQL queries in controllers/route handlers.

This changes ctx.db in context from *sql.DB to *db.Handler, which also has all query methods (in db/queries.go).

Todo

• [x] move queries from search.go
• [x] move queries from collections.go
• move queries from admin.go will not be done here
• move queries from application.go will not be done here

[modrzew]

@phpeter I verified that it works after my changes. Can you merge it?

[azertyfun]

I will probably end up implementing the rest as I fix the conflicts with my own branch