site: restrict allowed characters in CreateGroupRequest.Name to ^[a-zA-Z0-9][a-zA-Z0-9\-]*

Currently when creating a group we do not place any restrictions on the Name field in the UI.

However, when you then go to edit the group the restriction on the Name field blocks any further edits until the name is updated. We should enforce this restriction uniformly.

(Note: this only counts for Name, and not DisplayName).