Restrict uploads

[Arkshine]

I believe you need to use the subclass method to overwrite this computed descriptor correctly.

import discourseComputed from "discourse-common/utils/decorators";

api.modifyClass("service:composer", 
  (SuperClass) => class extends SuperClass {
    @discourseComputed
    allowUpload() {
      return super.allowUpload && canUpload;
    }
  }
);

EDIT:

Made a PR:

DEV: Modernize `modifyClass` calls, use tranformer, and add tests