Change database backend from MySQL (MariaDB) to PostgresQL

Created on November 16, 2017. Copied from redmine (https://bugs.ligo.org/redmine/issues/5986)

PostgreSQL seems to the the preferred RDBMS for much of the Django community. A comparison here: https://www.digitalocean.com/community/tutorials/sqlite-vs-mysql-vs-postgresql-a-comparison-of-relational-database-management-systems.

Making the switch would likely be complicated. But if we can make the argument that it would significantly improve performance (speed and/or reliability), it may be worth it.

One point of note: the abstract base AutoIncrementModel would need to be reworked since it executes raw MySQL queries, which likely wouldn't be compatible.