classic-ui: added option for partial servicegroups #2740

* adds new config option "show_partial_servicegroups"
* same behavior as "show_partial_hostgroups" just for servicegroups
* some bugs on "show_partial_hostgroups" where found and got fixed
* calculating and displaying of hostgroups in general should be faster now

Got so bored at the bus on the way home from Linux Hotel.
Someone owes me a beer now ;-)

refs: #2740
Merge branch 'feature/add_partial_servicegroups-2740' into next

Conflicts:
	Changelog

fixes: #2740