Ability to search users

[Toastbrot236]

This PR does the following:

• Makes the level search category also include matching users in its responses for games
• Adds a separate user search category for APIv3
• Makes level search no longer include levels by users with matching name (since searching users themselves is possible now)
• Allows level category game responses to include users

Needs unit tests and real tests, otherwise I think it's done.

Closes #997.

[Toastbrot236]

Tested outside of unit tests now, works.

[Toastbrot236]

Aside from adding unit tests, I also adjusted pagination for the slots endpoint (incase a listing will have way more users than levels, to allow the client to still paginate the users), and the playlist hack kept getting in the way, so I finally moved it to ByUserCategory instead.

Also, does hint_start really have to be above 0, even if there are no more pages? Since there is a test covering that.

[Toastbrot236]

I just tested LBP1, 2 and 3, they don't seem to have any issues with the new behaviour (levels are paginated fine), and it seems like LBP1 ignores hint_start for all playlist requests so it doesn't break anything there either.