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
I thought the Agent is an osra user (i.e. an 'AdminUser').