Use a ServletContextListener to configure servlets

This paves the way for ShiroWebModule integration. As an added benefit
Guice is now instantiating our servlets for us (no injector.getInstance
calls in our code).

Testing Done:
./gradlew -Pq build

No new tests added - the existing JerseyServletModuleTests already
cover correctness of wiring.

Reviewed at https://reviews.apache.org/r/25593/