Make Group Slug Changeable

[lorenjohnson]

Make the Group Slug changeable. Initial version could just allow a change to an available slug and a subsequent iteration could provide some sort of temporary or permanent re-direct option from the old to the new slug to provide for links going to the old slug. In the version that didn't provide any re-directs there should be a disclaimer about the fact that it will break previous links when before saving the change.

See notes on the original request by Paul Cereghino's here on Building Hylo: https://www.hylo.com/groups/building-hylo/post/55075