Added new user profile field for items per page on filtered views. Can be changed in the user profile. Need to plumb in to the filtered pages themselves. References issue #543

Added a drop-down box to choose the number of studies displayed per page on the filtered html pages. I tried to implement Django's built-in pagination but couldn't make it work with the filters. References issue #543

Updated documentation to reflect new option to choose how many studies are shown on each filtered page. Also updated changes files. Fixes issue #543