Rename default content directory to mathb-content

The default content directory is named as mathb-data. Rename
this to mathb-content.
Remove unnecessary blank lines

There are a few unnecessary blank lines in bag.php and
mathbin.php. Remove them.
Show static preview when JavaScript is disabled

The static preview is displayed as a PNG image generated from
the input code. This PNG image is generated using pandoc and
convert. The image is created on the first requested to the PNG
preview image, and then cached in a cache directory. For
subsequent requests, the preview image is picked up from this
cache.