Create agent model & agent-sponsor association

https://osraav.atlassian.net/browse/OSRA-205

- create Agent model with required unique attributes `first_name`, `last_name` & `email`
- create agents factory
- Agent `has_many :sponsors`, Sponsor `belongs_to :agent`

OSRA-205
OSRA-233 OSRA-234 #Code-Review
It's an OSRA employee, yes. However, it seems that OSRA only want one true admin to have all powers, while others would be regular users with limited access. We settled on this implementation - for now, employees can be entered into the db and linked to sponsors as agents. Later, we'll implement authentication & authorization for them so that they can actually use the app.