Switch new Client to a Specific Group automatically

[yubiuser]

They are, but its' not shown

Improve Group management/Client by making group association for default clients visible

Unconfigured clients are automatically members of the group "Default". Going to Group management/Clients this association is not obvious. Known clients are presented in the drop-down menu but no group association is visible. Therefore I propose to change the way new clients are displayed at Group management/Clients. Remove the drop-down menu (but keep the possibility to add manually configured clients/subnets) but add the clients to the "List of configured clients" (title has to be changed). B…