popup: Adjust margin and shadow.
popup: Adjust margin and shadow.

Make the layout styling of Popup component more consistent:
* layout using material design margin values
* 'bottom' styling is not needed
* 'shadowOpacity' and 'shadowRadius' to distinguish the popup
  better from the background

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile