Add a dev module to run Gitiles from an embedded Jetty server

The various ways of running WARs from maven directly have proven to be
a headache and not work reliably on seemingly-identical environments.
Instead, reduce the amount of Maven magic to a minimum, and use
Jetty's nice embedding support to start a server serving only a
GitilesServlet from /*.

Change-Id: Ib07c5f94cb76e7cd0208096591e3e7cc6148f1ac