[#841] Enforce max-width on images in smaller screens

* also removes similar code in some layouts because it's no longer
  needed

* add height to poll image style attribute  so it overrides height: auto
  (but don't remove height attribute, needed for, e.g., email
  notifications)

Fixes #841
[#841] Force a max-width onto embedded content (iframes)

* adds two new wrappers to the generated lj_embedcontent HTML:
  .lj_embedcontent-wrapper and .lj_embedcontent-ratio

* .lj_embedcontent now has a width: 100%, which forces it to be as wide
  as its containing element

* .lj_embedcontent-ratio gives back the height at the original aspect
  ratio using padding-top (we know the given width/height so we can
  generate this dynamically)

* .lj_embedcontent-wrapper forces a max-width/max-height, so that it
  can't go bigger than it was originally defined, or bigger than
  system-limited width/height

* the generated content inside the iframe (embedcontent.bml) is given a
  width/height: 100%, so that it's as wide / high as we defined the
  outer iframe to be

Fixes #841
[#841] Add back image size limits on larger screens

only for Snakes and Boxes, and Trifecta -- this was the old behavior so
let's not change it
[#841] Add image handling to all dw-free layouts

(These are the layouts that override Page::print_default_stylesheets, so
they didn't get the change automatically)

Fixes #841.
Merge pull request #846 from afuna/too-large-media-bug-841

Handle large media (images, iframes)