create a longitudinal xray view

Rather than just showing one xray at a time, we need to be able to show a time series of xrays in some way. There could be tens of xrays for just one subject, with one or two xrays taken per day at the NICU. Say there are 30 xrays, then you wouldn't want to display all 30 xrays at once, but it should be possible to view more than one at a time, and it should be easy to scroll through all 30.

This relies on #16, because we need the app to have a notion of "the collection of xrays for the currently selected patient"

This also relies on #14 so that we have a place in the UI to put the longitudinal xray view.

Start by reviewing issues #16 and #14, then inspect the existing single-xray view and its surrounding UI. Done means the selected patient's xray collection can show more than one image and lets users conveniently scroll through a larger series without displaying all images at once.

Written by the indexing model from the issue text.