[controller] In all: bring implicit request -> EMP into scope

Preparation to convert services to take an implicit EMP instead of
the request.
[service] Abstract: Take implicit EMP instead of Request[A]

Clearer separation between service classes and web framework.
[service] Conference: Take implicit EMP instead of Request[A]
[service] Figure: Take implicit EMP instead of Request[A]
[controller] Account: bring Request -> EMP implicit into scope

And mark the request as implicit in all actions.
[service] UserStore: move findByEmail to other own methods

Separate UserServicePlugin implements from methods we provide
on top of that. All our methods should take an implicit EMP.
[test] UserStore: make implicit EMP globally available
[serivce] UserStore: list, findByEmail now take EMP implicitly