Implement StoryProgressController

[BenHenning]

Users will make progress on individual stories when they play them. There should be a system that can be used to record and retrieve this state based on the story being played (by ID) and profile ID of the user playing it. Note that this system is storing based on individual exploration being played in the context of a story.

The actual storage necessary here is just completed exploration IDs that can be cross-referenced with the exploration IDs contained in a story.

[BenHenning]

This depends on #15 and #14.

[BenHenning]

This does not block on #14 since profiles may not be implemented in the prototype. #14 will need to add profile support to this data store after it's completed.

[BenHenning]

This is blocked on #119.

[BenHenning]

NB #118 might actually include progress for chapters per the requirements of the topic page. Either that impl needs to depend on this to derive progress, or both should use a common source. Perhaps what we really want is a StoryController for the story page, and use an internal StoryProgressController to determine progress.

Or, the TopicController can be used to report summaries for topic & summary pages, but the actual progress is controlled by this controller. That may be a nicer way to model things for now.

[BenHenning]

This has been completed a while ago.