Make bridge pool assignments available on the metrics website

We plan to make sanitized versions of the bridge pool assignments available for analysis. We already discussed in legacy/trac#2372 (moved) how to learn about bridge pool assignments from BridgeDB. As soon as we have these assignment files available on a regular basis, we'll have to

• extend metrics-db to sanitize the original files in the same way as we sanitize bridge descriptors,
• write a parser to convert old BridgeDB log files to the new sanitized bridge assignment file format, and
• put regularly updated tarballs of these files on metrics.tp.o.

Once legacy/trac#2372 (moved) is done, we should open new tickets for these tasks, or just work on them as part of this ticket.

Child tickets: TicketQuery(parent=legacy/trac#2537)