Fix Bug 1087020: Demo detail page mobile view

Lots of overrides to make demo detail page more mobile friendly.

- Added clearfix() to mobile column declaration, this could break stuff site wide (though it is more likely to fix stuff...)
- Added some stuff to make .media RTL friendly and to display the media below the content if .media-reversed is added.
- Solved most of the problems by switching from the demos grid to the MDN grid (added column-container)