This updates Jetty to the latest 9.x version as of writing. The 9.x is
still running on Java 8. The update needs two code changes.

`SessionManager` was replaced with `SessionHandler`. This was documented
in the Jetty documentation.

Adding the `GitblitContext` to the `WebAppContext` will result in two
instances getting created, because the code was changed that prevents
instantiation the same listener class multiple times. (The second time
is when the web.xml is read.) Instead, it must be added to the servlet
handler of the `WebAppContext`. This results in properly adhering to the
changed internal startup flow.

Updating Jetty also resolves #1409.