Add hub features

[Groverkss]

Allow this bot to give invites to registered servers, essentially creating a hub for IIIT Discord Servers.

[GaurangTandon]

We'll need a flag is_public_server: true/false because not all servers using our Discord-CAS service our intended public (example, ug2k20 discord server).
The flag will be true when the server is intended to be joined by anyone in IIIT.