Add pipeline_instances/compare page
Show condensed data about pipeline instances side by side on "compare"
page.

refs #1978
Merge branch 'master' into 1978-compare-pipelines

Conflicts:
	apps/workbench/app/controllers/pipeline_instances_controller.rb
	apps/workbench/app/views/pipeline_instances/compare.html.erb
Add basic select-and-compare interface to pipeline_instances#index.

refs #1978
Fix crash when component has missing attributes (e.g., no job was run).

refs #1978