Send snitch_map with error reports

[bcyphers]

Sometimes it's hard to tell from error reports alone which hosts are causing certain 3rd-party domains to be blocked (e.g. #1880, #1882, #1883). When a user reports that a page is broken, PB could include snitch maps for all blocked domains on the page.

[ghostwords]

This conflicts with the protection from local/unsophisticated attackers goal of #266.

If we dropped snitch_map entries for domains that Privacy Badger already decided to block, we'd have no data to share.

If we obfuscated (one-way, per-user-salted hashes) the domains (leaving debugging aside, Privacy Badger just needs to know when it saw a domain track across three different website domains, not what those domains are), we'd have useless (don't know the user salt) data.

[ghostwords]

This is sort of related to #1299.

I think whenever it's a mystery, it's probably one of our mis-attribution bugs (probably #1997).

Having snitch_map data would indeed be helpful, but I don't think we need it. We already mostly know what needs fixing, plus error reports continue to trend down. Closing for now.