mw.ViewPageTarget: Move .mw-body-content down to .ve-ce-surface

Update VE core submodule to master (be38e90):

* be38e90 ve.ce.Surface: Remove font-size: 1em;

Follows-up I5818dce6265.

* When applied to ve-ui-surface, it applies to both of its children,
  namely ve-ce-surface and ve-ui-surface-overlay. The latter
  (containing things like context menu and inspectors) should not
  inherit styles intended for rendering of wikipage content by
  the skin.

* Move mw-content-{ltr,rtl} from an outer parent to a slightly
  deeper parent of the actual content. It's now on documentNode.
  Still high enough to apply to all content. This was to ensure
  it remains a child of mw-body-content, like in MediaWiki core.

* Updated padding/margin hack again. Since now the font-size
  applies on the surface, this once again means that now 1em
  on the surface != 1em on the documentNode, so we need the
  calculation again. Brought it back and this time documented
  what it is based on.

Bug: 64599
Change-Id: I9ee4582a6a0ec9e128f5c36a6dbf2adbd0b216a1