fix horizontal scrolling on 900 pixel wide devices

Horizontal scrolling is still possible because some HTML elements extend beyond the 900 pixel mark on the right.
Fixes:
Modify technique used for centering puzzle progress nav (create 100% wide div, move the border underline into a span that wraps the inline-blocks)
Conditionally modify width of container and span10 in the header and footer to guarantee that we don't exceed 900 pixels.  NOTE: this is only done for the pages where we host puzzles (under the @full_width variable)