[Enhancement] Rename Site.isOnline() to Site.isStarted()

The status 'online' is kind of misleading because it is not clear what it is in
difference to the status 'started'.

With this change, a site is either started and fully operational or it is not
started and thus not operational.
Update Javadoc to match with previous rename of method
Merge branch 'pr/basil/site-started' into github/develop

Conflicts:
	modules/weblounge-common-api/src/main/java/ch/entwine/weblounge/common/site/Site.java