Allow image and gallery posts to fill the whole browser

This allows posts with the image or gallery format to fill more of the
available window space. The max-width is set to 1200px but could be
increased. Text-based posts (standard, aside, link, and status) are
still kept to a readable 550px wide.
Allow attachments to also fill browser window

This completes the first pass at allowing images to be full width. See
#1