Data / Migrate legacy "Download Data" section into datasets

[furilo]

We should migrate the /descarga-datos section

• Create datasets for each category (a unique dataset with all budget/execution/invoices years)
• For Agendas/Declaraciones we already have the Getafe ETL (not finished: https://github.com/PopulateTools/issues/issues/982)
• Substitute current links to datasets links
• Check that the links uses a valid token (if needed)