Refactor the login function with users.authenticate
Refactor the login function with users.authenticate

I injected the user service so I can use the login function in the front
end. I currently don't know if I'll use the session data, or how I'll go
about managing sessions, esp. since logins might all be handled by cas,
but for now I think this should be fine.

joshpaulchan/web-response,joshpaulchan/web-response,joshpaulchan/web-response