Make study datatype selection checkboxes

This changes from the selection of specific processed data for a study and data-type to a checkbox for the datatype of a study. We are of course assuming that we have only a single processed data object for a datatype in a study, but since we are only processing with the default pipeline currently this should be a safe assumption. We will need to revisit this later though, and issue #215 is still open for it.

I also fixed a small bug with the javascript verification for removing samples in an analysis. 
Does the templating engine allow the value to be cast to `int` here? If so, recommend doing so here, and removing the cast in `_parse_form_select` above