Extend config for redact usernames

LewsTherin would very much like extended options for redacting usernames.

https://www.reddit.com/r/InboxRevamp/comments/3jvrdo/request_only_redact_specific_usernames_when/

> When exporting a message thread, instead of an "all or nothing approach" to redacting, it might be a nice idea to allow you to select which names should be redacted, and possibly allow you to edit the resulting name.
