Allow overloading ContextListener::refreshUser()

Allow overloading refreshUser() for the use case of doing something special with user providers.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n.A.
| License       | MIT
| Doc PR        | n.A.