Absolute paths for bookreader assets.
Hide useless buttons, fix two-page view.
Improved how the document loads images.

This requires a rebuild of derivatives. Essentially this creates
multiple sizes of every page and selects the appropriate one to show to
the user depending on zoom, saving bandwidth. Also uses JPEGs instead of
PNGs.
Merge pull request #444 from OregonDigital/feature/BookreaderImprovements

Bookreader Improvements