refactor bill MODS citation processing
add a media row to the bill page with the sponsor's photo, a thumbnail of the text PDF, and related bills and their thumbnails

also:

* now that most of the Related tab has moved to the media bar, change its name to Explore
* redo the bill page tab styling to have a white background, no border around the panes, and no extra padding because of the old border
* some bill help text is moved to the bottom of the page
* replace the old bill text thumbnails (static files generated by a Perl script) with the new thumbnail view at .../_text_image, which now only was used for og:image and twitter:image:src.
combine the Explore and Citations tabs on the bill page into just Explore, now that Explore is half empty
change how master_a's negative margin works to shift up the heading box and advertisement from a position: relative elements to the regularly positioned row above it so that we don't get the inverse empty space at the bottom of the element