Users shouldn't be able to create duplicate groups

[americanhanko]

Is your feature request related to a problem? Please describe.

Although a given Mastermind Hub instance should allow groups of the same name, individual users should not be able to create multiple groups of the same name.

Describe the solution you'd like

"Whoops! Looks like you're already a member of a group named Fellas."
"How about a different name like #{GroupPicker.random_choice}?"