Data discovery + preview + import to PG

[pdobacz]

For now I bundle these 3 large topics together, to see what options we have. Once separate subtasks crystalize I'll divide up.

I'm treating CSV as priority and Excel at the back of my head.

EDIT: CSV as priority but not bothering with Excel

Detail note: I won't be reusing the CSV import from desktop, which was done via reference, so this part needs to be done in pure JS (or PostgreSQL, if there's a way 🤔 )