Add output_is_persistent attribute to job model.
Add output_is_persistent flags to example pipeline template.
Propagate output_is_persistent flag from pipeline to job.
Make sure persistence links get added for persistent components, even
when reusing jobs that are not marked persistent or belong to other
users.
Indicate which outputs are marked "persistent" on pipeline provenance graphs.
Fix reversed input/output in dependency explanation.
Add/explain output_is_persistent flag in example pipelines.
Mark output as persistent at end of job, if Job record says to do so.
Merge branch 'master' into 1932-job-output-persistent
Merge branch 'master' into 1932-job-output-persistent

Conflicts:
	doc/user/tutorials/tutorial-firstscript.html.textile.liquid
	doc/user/tutorials/tutorial-new-pipeline.html.textile.liquid
	sdk/cli/bin/arv-run-pipeline-instance
	services/api/db/schema.rb
Fix some whitespace complaints.
Move "empty blob" magic number into a Collection class method, remove
some old unused code and commented-out debug statements.
Clarify c_already_finished comments.
Merge branch '1932-job-output-persistent'

closes #1932