Creates the LabelSequence resource (migration)

[#46033697] This creates a stock model and controller, a bunch of empty tests, fleshes out the model's relationships and adds the migration.
Spec test for LabelSequence model [#46033697]
Basic UI for adding LabelSequences to Pages.

[#46033697] This ignores the question of the proper way to handle the LabelSet in question, which should actually be empty and invisible (because the student is going to fill it).
Adds the is_for_users attribute to LabelSets

[#46033697] This lets us designate if a LabelSet is intended for the author to place labels on a graph, or if it's intended to hold labels placed by the user at activity run time.
Don't serialize LabelSets intended for users

[#46033697] When an Activity serializes, the labelSets attribute of the hash won't include any LabelSets where is_for_users? is true.
Label sequences LabelSets must be is_for_users.

Also, if the author doesn't initially designate a LabelSet, one is automatically created. [#46033697]
Merge branch 'label-sequences'

Conflicts:
	app/models/activity.rb
	db/schema.rb