[#1427] Reworked admin scopes support.

Requalified users.is_admin flag as legacy accessor to admin actions in case token lacks admin scope(s).

test/web/admin_api/admin_api_controller_test.exs

@@ -1537,7 +1537,8 @@ defmodule Pleroma.Web.AdminAPI.AdminAPIControllerTest do
         |> assign(:user, user)
         |> get("/api/pleroma/admin/reports")
 
-      assert json_response(conn, :forbidden) == %{"error" => "User is not admin."}
+      assert json_response(conn, :forbidden) ==
+               %{"error" => "User is not an admin or OAuth admin scope is not granted."}
     end
 
     test "returns 403 when requested by anonymous" do