Add verification of all e-mail addresses in a public key block

Right now, it is possible to verify oneself via various means like Twitter, Github, and others. It would be very nice if the e-mail addresses given in the uploaded key would also be verified as valid. Of course, revoked e-mail addresses and such user IDs without an e-mail address should be skipped.

The issue names no files, tests, or entry points. Start by locating the code that processes uploaded public key blocks and user IDs, then determine how existing identity verification is performed. Done means non-revoked user IDs containing email addresses are verified, while revoked addresses and IDs without email addresses are skipped.

Written by the indexing model from the issue text.