Clicking "Join" in a subplebbit doesn't need to load and also doesn't need a "success" popup

[estebanabaroa]

Subscribing should be close to instant and the user feedback should be that the button changes from "Join" to "Joined" so no other indication should be needed. A success popup is not needed.

The useSubscribe hook should be use https://github.com/plebbit/plebbit-react-hooks/#subscribe-to-a-subplebbit