Export pipeline as JSON feature

Feed view should have a button "Export" which creates a JSON representation of the selected DS plugins as part of a reproducible workflow, aka pipeline. This JSON file can be uploaded to the ChRIS store via POST request. Example:

https://github.com/FNNDSC/CHRIS_docs/blob/master/pipelines/fetal_brain_reconstruction.sh

A corresponding feature in ChRIS_store_ui is supporting import of these JSON representations of pipelines. https://github.com/FNNDSC/ChRIS_store_ui/issues/224

There should already exist an API endpoint which, given a plugin instance ID, saves a snapshot of the plugin instance and its children as a pipeline. Essentially we just need to connect this endpoint to a button.