Postgres support v3

[NodudeWasTaken]

My third attempt at good postgres integration.
I do have some gripes about how it was made.

• It has alot of duplicate code because postgres and sqlite need different sql syntax.
• Using group_by is a little annoying, but using distinct in postgres is simply painful.
• Some of the filter builders could surely be shared.
• I had to use a dev version of goqu to support "returning".
• Fingerprints work a little weird in postgres because the codebase expects it to return the correct type from sql.
• Perhaps the interfaces could work better with a shared Interfaces() function.
• Basename in postgresql has questionable code.
• Some SQL optimizations on the Scene and Images table could be backported to SQLite i think.
• Custom fields are already json in postgresql, but not yet for sqlite.
• I had to add a type migration for the custom fields to know how to migrate them and parse them.
• I made a savepoint creator to avoid sql lockup from postgresql.
• It could benefit from some setup instructions 😝

Overall it works very well though, i've been running it for some time now.