Figure out auto-update of public suffix list
The "public" option for the rate limit mechanism depends on having a copy of https://publicsuffix.org/list/public_suffix_list.dat. Figure out the operational implications, how to download a copy on initial install, and do periodic updates (appropriate frequency is once a week or every other week). Would need either some code in the sigsum-log-primary server to reload the list, or periodic restarts to pick up new config.