Restore the group#slug URL

- this roughly reverts the commit:
'Use hierarchical URLs for groups'

0da4a216753b6719d08398fcd5b915989599235a
Rebuild the group slugs

- Take advantage of friendly_id's ability to build
slugs based on a hierarchy: #slug_candidates

- Ensure that slugs are recreated when a group
name is changed (does not apply to changing the
name of a parent that might have been created 
by the hierarchy)
Update the group slugs and restore the index

- Note that this migration fails when run in a
batch with this error:
PG::FeatureNotSupported: ERROR:
cached plan must not change result type

- This appears to be caused by executing sql
in the previous migration that adds ancestry
to the group

- If the migration fails, it works the next time