code: public boolean allowsSignup() {
          Class clz = getClass();
          return clz.getClassLoader().getResource(clz.getName().replace('.','/')+"/signup.jelly")!=null;
      }
docs: Returns true if this {@link SecurityRealm} allows online sign-up.
  This creates a hyperlink that redirects users to {@code CONTEXT_ROOT/signUp},
  which will be served by the {@code signup.jelly} view of this class.
  
  <p>
  If the implementation needs to redirect the user to a different URL
  for signing up, use the following jelly script as {@code signup.jelly}
  
  <pre>{@code <xmp>
  <st:redirect url="http://www.sun.com/" xmlns:st="jelly:stapler"/>
  </xmp>}</pre>