MIQA server sessions endpoint doesn't use 'imagepattern' correctly

[curtislisle]

the getSessions endpoint in the MIQA server doesn't find datasets if the filename doesn't end with .nii.gz. In particular, the imagePattern should be used here instead of the hard-coded regex:
https://github.com/OpenImaging/miqa/blob/57fdc5fee5dff86a01539a1055d3442637ff1785/server/miqa_server/session.py#L117

[scottwittenburg]

When I wrote the json data model, I intended to put imagePattern there instead of nii.gz$, and thought I had done so, but must not have 🤷. I think the fix is as simple as just making that replacement and testing that it behaves as expected.