Provide a means to reset unblock history

[jsha]

Sometimes people want to bulk unblock a lot of accounts, but not necessarily have those accounts whitelisted in the future. Either:

1. Batch unblocking from the UI shouldn't whitelist accounts. Probably not good because people may use it for one-off unblocking and it would be surprising for this to act different than on Twitter.

or

2. There should be some way to say 'Ignore all unblocks prior to now', effectively resetting the unblock whitelist history.

or

3. Once #116 is resolved, unblocks caused by 'unsubscribe and unblock' should not count as whitelisting.