Support PostgreSQL

[La0]

This brings support for PostgreSQL into Treeherder, while keeping it compatible with Mysql:

• add postgres:15 in docker compsoe stack
• add psycopg2-binary python requirement
• support postgres in settings (only mysql was compatible with force-applied settings)
• support both postgres & mysql in the handwritten migrations (mostly indices).
• run unit tests on CI against mysql then postgres

To test it locally:

docker-compose up -d postgres
export DATABASE_URL=psql://postgres:mozilla1234@localhost/treeherder
./manage.py migrate