Reintroduce postgres support

[justjanne]

In Short

• [x] Validate and test the changes
• [x] Ensure postgres support in Seahub and Seafile-Server as well

Related

https://github.com/haiwen/seafile-server/pull/560 https://github.com/haiwen/seahub/pull/5196

Rationale

Many users (me included) use Seafile with PostgreSQL databases. Seafile has supported Postgres databases with small patches for many years, but during a recent db backend rewrite that functionality got removed. My own deployment has been stuck on 7.0, with no update in sight.

Due to that, I’ve decided to implement Postgres support back into Seafile. If it doesn’t get merged, I’ll have to maintain a custom fork with downstream patches, which I’d like to avoid.