Live deploy as a nightly version

I've added the live deploy option as a nightly version. This future is enabled by setting the watch flag to true.

review @devoncarew @ussuri
This is subtle and probably won't happen often, but: theoretically, the value of the flag may change while the app is running. As you have it now, that won't make live deployment available until you restart the app. You can fix that by moving `if (SparkFlags.liveDeployMode)` to right before `return localPrefs.getValue("live-deployment")`.