WIP on ensuring derivative DataSets copy properly.

[#56320646] Currently the derivative_of property gets dropped. This is a stab at how to fix that in the model. Needs testing.

https://www.pivotaltracker.com/story/show/56320646
Merge branch 'master' of github.com:concord-consortium/smartgraphs-authoring
Refactoring DataSet <=> Pane relationships: PredefinedGraphPanes
WIP on LinkedAnimationPanes - creation, initial relationships
Linking LinkedAnimationPanes and DataSets
Moving SensorGraphPanes to DataSetPane join
PredictionGraphPanes get the polymorphic DataSet association
LabelSet polymorphic association with GraphPanes
Database cleanup from old branch
Fixes to get LinkedAnimationPane creation working
Fixing views to reflect new DataSetPanes
Supporting LinkedAnimationPane serialization
Updating generator with LinkedAnimationPane
Restrict LinkedAnimation DataSets to derivatives
WIP on LinkedAnimationPane testing
Improved spec tests
Exported DataSets have null derivativeOf and piecewiseLinear attributes

This fixes a bunch of failing Cucumber tests by correcting their expected JSON output.
Fix page creation step for Cucumber

For some reason the text of the link changed.
The apostrophe gets entity-ized and breaks the test
Hiding WIP on DataSet derivativeOf copying