[feature] Make OIDC admin groups configurable (#1555)

This removes the current default of checking for membership of the admin
or admins group and makes it required to explicitly configure which
groups should grant admin access, if any.

Relying on the implicit default of admin or admins is potentially
dangerous as that group may contain a different subset of people that we
may wish to grant admin access to GtS. This is probably not an issue for
a single-person instance, but for a community instance different admin
groups may exist in an OIDC provider for different applications.

I'm explicitly opting for not defaulting the value of oidc-admin-groups
to admin,admins because I think it's better for those things to be
explicitly configured.

internal/api/auth/callback.go

@@ -284,10 +284,15 @@ func (m *Module) createUserFromOIDC(ctx context.Context, claims *oidc.Claims, ex
 	}
 
 	// check if the user is in any recognised admin groups
+	adminGroups := config.GetOIDCAdminGroups()
 	var admin bool
+LOOP:
 	for _, g := range claims.Groups {
-		if strings.EqualFold(g, "admin") || strings.EqualFold(g, "admins") {
-			admin = true
+		for _, ag := range adminGroups {
+			if strings.EqualFold(g, ag) {
+				admin = true
+				break LOOP
+			}
 		}
 	}