More sophisticated defaults for tasks

[manthey]

By default, tasks with certain types of outputs get pre-populated with an output file in the current user's "Private" folder named as {analysis-name}-{output name id}.{extension} (see https://github.com/DigitalSlideArchive/HistomicsUI/blob/master/histomicsui/web_client/views/body/ImageView.js#L484). It'd be nice to specify if the output should be in the private folder or next to a source file.

[manthey]

This really should just support a <default> value with the existing jinja-ish templating that slicer_cli_web does. If we have a default on a girder resource, this should either be a complete or partial girder path; for outputs, this could work if the templating is only in the final path element, or we need to percolate through a string rather than a backbone model throughout.