Add most-demanded items

[karashiiro]

It's actually a bit unclear what this will entail. It can either be interpreted as a measurement of what items are uploaded the most, or as a measurement of what items are most searched-for.

If this ends up being the former, this only requires the definition of a new extra data document and an endpoint to go along with it, along with a panel on the front page to show that data to the user. I think the right sidebar is getting a bit cluttered, but I'm not sure how to cleanly position an element in the middle of the page that only fills a third of the width and allows other elements to live in the same row. If all else fails, we can just put it in the right sidebar and clean it up later.

If this ends up being the latter, this will additionally require a new POST endpoint to be made on the server for the frontend to access. Access security shouldn't be required, as I'll be hiding the endpoint from the public in nginx.

[karashiiro]

Total gil trade volume per unit time (day?) could be a good metric for this, and we already have history entries timestamped, so it should just be a matter of querying for the most recent sales. That does remind me, though, that the database is structured a bit strangely, and fixing that should be a long-term goal.

[karashiiro]

Basic implementation done with 65b7ae2, needs basic testing before going live.