Empty the database during Postgres backup restore

The expected behavior during a GitLab backup restore is to overwrite
existing database data. This works for MySQL because the output of
mysqldump contains 'DROP TABLE IF EXISTS' statements. pg_dump on the
other hand assumes that one will restore into an empty database. When
this is not the case, during the restore with psql some of the data will
be skipped if existing data is 'in the way'. By first invoking `rake
db:schema:load` during a Postgres GitLab backup restore, we make sure
that all important data is correctly restored.