Some labels confer automatic public visibility

[osteele]

Some labels make their events publicly visible.

This could be implemented something like:

• Add an optional visibility: "public" field to labels
  • Add public = BooleanField() to class Label(Document)
  • Add "public": fields.Boolean to label_model in label_resources.py
• Modify An unauthenticated client should not receive non-public events #75 to compute event visibility based on label visibility.

Computing event visibility could happen after the mongodb query, something like:

public_label_names = {label.name for label in db.Label.objects(public=True)}
results = [event for event in results if not set(event.labels).isdisjoint(public_label_names) ]

This isn't terribly efficient, but it's probably adequate, and if the server is getting hammered by outside visitors we probably want a more generic caching layer anyway.

This task should also propose the set of label (Featured? Academic Calendar? Collaboratory?) that have public visibility.