Add custom field support for Discord usernames

[KazWolfe]

This plugin should auto-fill a user's Discord username (in format @MyUsername#1234) into a custom field on a user profile.

The field selected for this should be configurable from the admin panel, and not selecting a field should disable this feature.

The username in this field should be changed/updated on every subsequent login if necessary.

[featheredtoast]

I get where you're going with this, but I'd have to give this some thought as to the best way to go around implementing something like this.

My main concern here is that I'm not sure that having this plugin expose a discord username by default is a good way to go.

[Reyowski]

Not exposing the username publically, but only for admins seems to be a good idea.