Story 1621: Fixing VE content area typography/design for mobile

Lots of changes to make sure that page content looks the same in VE
as it does in regular page view on mobile:
* Add VisualEditorOverlay.less to override some VE specific styling
* Change h3 selector to more specific .mw-mf-overlay-header in
  OverlayNew.less so that it doesn't apply to regular h3s in VE.
* Move thumbcaption styles out of thumb selector since thumb
  captions aren't inside thumb divs in VE :(
* Add rellink to hatnote styles since it should be styled the same
  as dablinks.
* Change margin to padding in hatnote styles, so that it doesn't
  affect position of floated elements below them.
* Changing all #content selectors to .content since the ID doesn't
  exist in VE.
* Make sure appropriate header styles are applied in VisualEditor.

Change-Id: Ie5084349a5f5edc5f9a05c93bdcf9786a07b49f6
Merge "Story 1621: Fixing VE content area typography/design for mobile"