Automatically pull in bio and location from Twitter

My Keybase bio and location are the same as my information in Twitter, so it would have been helpful for that data to have been filled in automatically as a default because I hadn't yet set any other bio information.

I think doing this will help fill out the community more, as there will be fewer users with totally blank profiles.

It would also be cool to let @usernames in the Keybase bio link to another Keybase user page like it works on Twitter.

The issue names no files, tests, or entry points. Start by locating the profile fields and the Twitter-related import flow, then determine how bio and location defaults should be populated and how @usernames should link to Keybase profiles. Done means both requested behaviors work without overwriting other profile information.

Written by the indexing model from the issue text.