[App Directory] Allow Developers to set the category of their app

Currently, the category of bots inside the app directory is automatically set by Discord, not by the developers. It seems to be an automated thing based off of it's description, as I've seen a couple of bots (including my own) being falsely marked as one category when being nothing related to it.

(Image link cus Discord's image uploader doesn't work)

The best fix for this in my opinion is to just use the tags that are already inside the developer dashboard, as these are set manually by the developer already, and wouldn't require any extra work to swap to as they've been there for ages. Would also give further consistency to the app directory + bot itself.

(Image link cus Discord's image uploader doesn't work)