Problem: Support Postgres as a DB backend

[pancho-villa]

Please describe the problem you'd like to be solved
Our lab has limited resources but we already are running a multi-tenant installation of postgres, is there any reason Archivematica can't use this DB instead of standing up mysql on its own?

Describe the solution you'd like to see implemented
As a sysadmin I'd like to change a config with my connection string to postgres. I briefly looked at the code, and this appears to be a django app, which already supports it. I understand if there are issues not supporting it because of other integrations like elastic search.

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