Allow a server's default Notification Settings to be configured by a user-by-user basis

Currently for Discord servers, the default notification settings can be configured by the admin or moderator for that server only, which means for newer unmanaged servers (for the purposes of a one-time-event or such) most admins do not go through the motions of configuring those settings themselves.

Users should be also given an option to override a server's default notifications settings when joining new servers with their own preset defaults.