Persist Sprint state to DB

[tobiasmcnulty]

I'm thinking we could do something fairly lightweight, such as serializing the sprint to DB after every change, and then reading it back if the process is restarted for an upgrade or what have you (the main goal being to avoid destroying all sprints every time we release a new version).

The data model could be as simple as a single sprints table with id INTEGER, sprint_id VARCHAR(32), sprint_name VARCHAR(255), and sprint_data JSONB fields.

Related: http://www.ubazu.com/restore-process-state-in-elxir