Fix up web.xml config

Add missing WebServlet annotation to JavascriptConfigServlet class, for
consistency.
Remove redundant configuration items from web.xml, as on Tomcat 7 with
default settings these cause the following error:
java.lang.IllegalArgumentException: The servlets named
[JavascriptConfigServlet] and
[net.socialgamer.cah.servlets.JavascriptConfigServlet] are both mapped
to the url-pattern [/js/cah.config.js] which is not permitted
Fix gamelist refresh on leaving game

Game list update was skipped if the game list wasn't visible.  Fix up a
couple of cases when switching back to the game list the update was
called in the wrong order.
Add some error handlers for leaving games.

Trying to leave an invalid game always succeeds.  Trying to get game
info about an invalid game (that you think you're still in) makes you
leave it.