Set up OONI's MetaDB on polyanthum

As part of legacy/trac#32740 (moved), we need to sync OONI's test results with BridgeDB's SQLite database; in particular its BlockedBridges table. Over here and here, hellais suggested to set up a copy of OONI's MetaDB and have it sync with their canonical database. We can then use our local copy on polyanthum to update BridgeDB's SQLite database.

Instructions for setting up a MetaDB are available at: https://github.com/ooni/sysadmin/blob/master/docs/metadb-sharing.md