Migration: Give options to choose which contacts are migrated

[goobertron]

One thing I would like as part of a migration feature is the ability to be able to decide which contacts are migrated. Obviously with people I am sharing with, I can unshare before migrating. But it is harder with people who are sharing with me but with whom I’m not sharing. One reason for someone to migrate is to escape stalking or other harassment, so some means of doing this would be good. Again, probably outside the scope of this particular PR, but I mention it here so it’s at least in the discussion.

Since writing this, I've noticed in a second test that the 'only sharing with me' contact on that account was not transferred. I don’t know whether or not this is intentional. I had ‘ignored’ that account before exporting my data, so it might be that this makes a difference to whether or not the contact is imported. I am happy in my personal case for ‘only sharing with me’ contacts not to be transferred, as I have no interest in having followers, but for some people, who post a lot publicly and/or use diaspora like a blog, it would be a big disincentive to migrate if they would lose all their followers in the process.