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
No, but I'll change them to something even more fake :)