Build enhancement: Add WAR distribution

### Feature requests and proposals
Propose to add WAR distribution to make Metabase be installed into different application servers.

Here is the link on Stackoverflow: [How to create WAR](http://stackoverflow.com/questions/33135667/can-i-make-lein-ring-server-headless-run-on-a-specific-servlet-context)

:arrow_down: Please click the :+1: reaction instead of leaving a `+1` or `update?` comment

Start by reading the linked Stack Overflow discussion on creating a WAR for a Leiningen/Ring application. Then inspect Metabase’s existing distribution and build entry points to determine the application-server packaging scope. Done means a documented, reproducible WAR distribution that can be installed in different application servers.

Written by the indexing model from the issue text.