unique constraint added for edges is not sufficient

[lolopinto]

It only works for edges that are in their own table

TODO in code still needs to be addressed: https://github.com/lolopinto/ent/blob/master/internal/db/db_schema.go#L491

partial indices probably what we want here: https://www.postgresql.org/docs/current/indexes-partial.html

never finished functionality when I added it...