Use `--arrowpanel-` variables for tor circuit panel spacing

Currently we set some of the spacing using fixed values (4px, 1em, etc) rather than the available --arrowpanel- variables. We should use this to be consistent with firefox and ensure the "New Tor circuit for this site" text start-aligns with the "Tor Circuit" text.

Also update this comment:

#tor-circuit-alias:not([hidden]) {
  /* Same margin as .panel-header > h1 */
  margin: 4px 0;

which is outdated.