precautions against pacUrl instability (getlantern/lantern#618)

- back out 447fe71269f533c5bb9f869ca323acb8c76a68b5 which backed out
  https://github.com/getlantern/lantern-ui/commit/024e72c5#diff-5
  to catch "10 $digest() iterations reached" errors thrown by Angular
- ditch the collapse and always show pacUrl in Advanced settings to avoid the
  "10 $digest() iterations reached" error if pacUrl changes mid-animation