Force thumb and frameless images to default size in DM

* Force wiki-configured default size for thumbnails and frameless images
  in the DM. This is done because at the moment Parsoid's output is of
  Wikipedia's default size rather than the local wiki's. The size is
  adapted if needed, directly in the DM.

Bug: 62013
Change-Id: Ia32253e8620c0cd2cbf7334342113c8585a7868a
(partial cherry-pick of 89aecd54bad16ba1d4a6086b661afd45fe3ad2d6)