Add proposal target model, api endpoints, and tests

• Added a Target entity
• Target has an epoch which defaults to J2000 (as per Northstar) for future compatability, not shown in UI (for now)
• Target does not have a coord system and implicitly defaults to equatorial coords (as per Northstar), maybe something for future consideration but not needed for now
• API only exposes a proposal/:id/targets endpoint as fetching targets regardless of which proposal they belong to seems unnecessary. So it's not possible to fetch /targets