Twitter default egg picture overrides Gravatar image

After adding a Twitter proof for my new Twitter account, I was dismayed to discover that Keybase pulled in my default, Twitter-provided Twitter image – [this egg](https://s3.amazonaws.com/keybase_processed_uploads/a37db6b09e47a4bae6edef018078a605_200_200_square_200.png) – instead of my somewhat more relevant Gravatar picture.

(This issue is basically a stronger version of #1195 – I think in general it’s not that bad to pull the image from Twitter, but it’s pretty silly that Twitter’s default image is used if the user hasn’t provided a custom image.)

No files, tests, or entry points are named. Start by tracing how Twitter proof images are selected and how fallback profile images are handled; done means Twitter’s default egg is not used when a Gravatar image is available, while custom Twitter images still work.

Written by the indexing model from the issue text.