Make BridgeDB take into account its BlockedBridges table

BridgeDB has a table called BlockedBridges in its SQLite database. The table is empty and currently unused but we are extending it over at legacy/trac#34154 (moved).

BridgeDB already has code to deal with bridge blocking (e.g., here) but we still have to make it process BlockedBridges and take it into account when handing out bridges.