Speculative fix for name disconnect between json and GraphLabel model
Associates existing LabelSets and PredefinedGraphPanes.

[#46033683]
Refactoring PredefinedGraphPane model spec
Adding label_sets to to_hash (with rescue if they're not there)

[#46033683]
Test serialization of PredefinedGraphPane LabelSets

[#46033683]
Tweaking UI for addition of LabelSets to Predefined Graphs

[#46033683] This includes a link out to the "new label set" form. I'm unsure of the utility of this; on the one hand, it seems useful to be able to add a label set "on the fly", that is, if the label set wasn't created at activity creation time. On the other hand, once this new label set is created, the user is left without a simple route back to their PredefinedGraphPane, which seems to me to be moving the inconvenience to a different part of the process rather than removing it entirely.
Merge branch 'add-labelset-to-pdgp'

Conflicts:
	db/schema.rb