Move mediawiki database to postgres

[frankduncan]

Currently we deploy some of our competitions (100Change2020, EO2020) on mysql, specifically mariadb, and we want to move them to postgres. We would need to migrate running systems in production and staging, which may be challenging. Also, all the extensions need to be checked that they have postgres sql commands. The ones we developed do not at this time. So when this issue is undertaken, tech debt items need to be added to those as well.