Add handling of private apps data

[Lifeismana]

Moved fetching the store token to its own function to dedup but i'm not sure if storing it in local storage should be kept, do we trust users enough to not get it stolen?

I've tested FetchPrivateApps but not SetAppPrivate

I haven't gated this behind an option since, depending of what's made of this, the fetch would fit under "Enable extra filtering on sales page and bundles page" and the set under "Wishlist, follow, and ignore products on the Steam store directly from SteamDB"

[xPaw]

I don't want the highlighting to be too overloaded (especially with extra requests), and I'm not entirely sure there's much value for knowing private apps on steamdb site.

[Lifeismana]

I also think that highlighting them isn't that useful (that fetchPrivateApps is probably not placed well in regard to that)
the only value i really see is show every private app in 1 place since it's not something that Steam does, maybe with a new filter "show only private apps" here https://steamdb.info/sales/?displayOnly=OwnedGames (although also having that filter under the default sale page might be weird)