Lookup and fetch GPG keys when adding emails

[François Marier]

When a user adds a new email address, we should lookup their GPG key on a local keyserver. If one or more keys are found, then prompt the user to click on the one he/she wants to use (or click "don't use a GPG key").

When a key is selected, Libravatar should store it in the DB and then all emails to this user should be encrypted using that key.

There should also be a way to add/remove GPG keys from the user profile page.

Finally, the keys should be updated frequently (on cron) and revoked keys should be removed from the database.