Rename agent to user

**Contains views code from #142**

https://osraav.atlassian.net/browse/OSRA-243

To reflect the fact that agents dealing with sponsors are just one type of OSRA volunteers, and that all volunteers and application users will not necessarily be sponsor agents, the Agent model needs to be renamed User.

OSRA-243 #Code-Review
its difficult to follow the flow but I think that email and username are not indexed. They need to be indexed with `unique: true`