Allow to select a group as a rule source or destination

[martinmacko47]

• Now a user may enter only an explicit IP address or IP network range
• Instead, the source and destination fields should be comboboxes with a list of defined client groups.
• If the user selects a group, the source/destination value should be saved as a group in DB. It must not be saved just as a list of current group users. We need to maintain the rule source/destination value dynamic as group clients change.
• User should still be able to enter an explicit IP address or an IP network range for the source/destination
• User should still be able to enter an empty value for the source/destination meaning any source/destination.