Sponsorship models

First PR of PT Story _Link Sponsor with Orphan_ https://www.pivotaltracker.com/story/show/78039462

- create models Sponsorship & SponsorshipStatus
- create factories and model tests for both
- write new Initializer method to set a new Sponsorship's status to 'Active'
- seed db with 'Active' & 'Inactive' SponsorshipStatus
- fix dumb copy-paste mistake in sponsor_spec
This table will be tiny but if we are searching on any of these columns it would be good practice to index them.