Modified Models to accommodate a feed

Checkin
• Added a default scope
• Added field validation
• Convenience method that finds checkins from projects followed by a
provided user.

Project
• Added field validation

User
• Defined the `feed` method that gets checkins in projects the user
follows
Added Home Feed

Paginates a home feed.

Right now, it doesn’t populate your own posts in the feed. This needs
to be fixed.