Return typesafe object to stapler

JavaScript method should return typesafe object without converting to
string.
Rename methods and fix wait logic

Change method name from doStart/StopConnection() to doWakeup/doSleep().

In those methods, change below:

* Get connection status from GerritConnectionListener instance
* Return changed connection status as json object
Remove tabs from javascript file

Convert to 4 spaces.
Fix endpoint to wakeup/sleep
Remove connection control form from server config page

Connection control feature is replaced to server list.
So remove form.

GerritServer#doConnectionSubmit is no longer used. So remove it.
Avoid to generate wrong link

If frontEndUrl is empty, wrong link is generated.
This patch avoids it.
Fix checkstyle error
Change redirect page when submitting server config

When save button is pushed, global config and job config are redirected
to upper page. But gerrit server config is stayed in the same page except
when server name is renamed.

This patch always redirects upper page from gerrit server config.