Refactor the GerritManagement page and add null checks.

Changes:
1.The GerritManagement page only shows the list of servers now.
The configurations for each server have been moved to GerritServer/index.jelly.
2.Add null checks for methods calling PluginImpl#getServer.
3.Use a LinkedList instead of ArrayList for GerritServers in PluginImpl.
4.Initial (yet disabled) code to eventually support server renaming
(See TODO comments in GerritServer#doConfigSubmit).

Change-Id: I1917c320b16bfe7270773ea4a851712b2350f573