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"