Added div for histogram and style for it.
Added printing of histogram json.
Added cache age histogram!
Avoided printing graph if no data. Added date histogram was generated.
Merge branch 'master' of git.curoverse.com:arvados into 2638-add-cache-age-disk-usage-histogram-to-workbench
Added some documentation and renamed variables after Tim's review.
Merge branch 'master' of git.curoverse.com:arvados into 2638-add-cache-age-disk-usage-histogram-to-workbench
Merge branch '2638-add-cache-age-disk-usage-histogram-to-workbench' closes #2638