Draft: Constrain group name: unique, url-safe, not 'all'

In preparation for upcoming feature extensions to the group model, we constrain the group_name to be a unique, url-safe string. Additionally, 'all' is reserved to prevent confusion.

This MR add a constraint to the mongo document UserGroup. This seems to lead to errors as the old, non-unique index seems to persist and have a name conflict with the new one.