Add overview of free rooms / "Find a free room" function

There are often situations in which someone quickly has to find a free room.

added part::backend part::frontend source::customer type::feature labels

added workflow::discussing label

marked this issue as related to #66

Should be implemented together with the room reservation feature (#66)

mentioned in issue #120

marked this issue as related to #120

added prio::1 label

assigned to @hansegucker

marked this issue as related to #165 (closed)

mentioned in issue AlekSIS#58

unassigned @hansegucker

added prio::3 label and removed prio::1 label