Implement user state save/load

[bunsenmcdubbs]

• Create column/table for storing user state
  • via database migration
• Add get/set functionality to model/user/user.go
  • Add unit tests
• Add handler in controller

API routes (proposed in #58):

Route	Method	Description	Request	Response
/user/{id}/state	POST	Save client state for the current user	{state: <state obj>}	{status: "ok"}
/user/{id}/state	GET	Get client state for the current user	-	{state: <state>}

If #57 (user authentication middleware) is not yet complete, this can be initially implemented with a static user and wait until that feature is merged.