Agent views

Part 2 of https://osraav.atlassian.net/browse/OSRA-205

- register Agent with ActiveAdmin
- create New, Edit, Show & Index views for Agent
- add Agent field to Sponsor New/Edit form
- page for sponsor assigned to an agent has link to agent page; agent has links to assigned sponsors

- refactored Agent to use `agent_name` in place of `first_name` & `last_name` - facilitates searching and sorting
- refactored Cucumber step for checking links to index pages

OSRA-235 OSRA-236 OSRA-205 #Code-Review