Renku 2.0: Update the new group form

[leafty]

1. Update the help text for the slug -> "A short, machine-readable identifier for the group, restricted to lowercase letters, numbers, and hyphens."

2. Add an indication for the slug that it determines the group URL, e.g. "Determines the group URL. You group will be located at https://renkulab.io/v2/groups/some-group"