Keep track of and deal with censorship by exit node ISPs

We recently got a report by someone mentioning that

D55D5F6C5EB8AFA861A455BC44F8E5FD766D5E99
424BF86927E80D916589BB12248BD468BB470684
6739A6A7147E03350BC6552B8D72125DF3742592
5F6B1DF8C9FA561EC11670C455A4A9188D7F96D9

block various sites like yandex.ru or yandex.ua, mail.ru, vk.com etc. claiming this is essentially censorship by ISP. We don't have many exits at the same ISP but checking with B62DE0BD62FBFFC3398014A73839C2E49A6D4CC0 shows the same problem.

ISPs in Ukraine are not the only ones doing this kind of censorship, we've seen similar issues in other countries, though.

This ticket is the parent ticket to the following tasks:

• find a place where we keep track of that information
• figure out means how to scan for this kind of censorship
• think about how to handle those exit relays

Note: for a user things do not look differently if websites block Tor users. Thus, one could be inclined to conflate both problems. However, we should not do so as websites not wanting to have Tor users vs. ISPs interfering with Tor connections are different problems requiring very likely different solutions.

/cc @gus @cohosh @arma