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]