Pull avatar images from key servers

When signing up (and later), it should be possible to pull some personal information from key servers. One such thing could be an avatar image.

For example, my own key as well as that of other people in my key ring contains such an image.

Relates to #2343

The issue names no files or tests; start by reviewing related issue #2343 and the signup and key-ring flows that fetch information from key servers. Clarify which key servers and personal information are supported, when avatars are fetched, and what successful avatar retrieval should look like before implementation.

Written by the indexing model from the issue text.