Pipelines with data files and pipelines diff

This:

• removed DataFile.handle
• adds optional DataFile.pipeline reference
• rewritten WebDAV patient syncing to get inputs and pipeline files from FileStore
• produces diff view for two pipelines (all pipeline files, not just a selected step)
• compares only 'comparable?' DataFiles.

I propose to leave diffing at pipeline level at the moment - we have only two comparable (i.e. not binary) files that could be diffed (and even then, one of them is a large one liner - a very inefficient file format for a diff), so it is probably better to just compare whole pipelines.

There is one temporary solution here:

• ComparisonsController.show performs Patient sync every time it is run; this should not be required after proper patient data syncing is in place.

Fixes issue #202 (closed) Partially addresses issue #178 (closed)