support discovery of public keys for legacy providers
I can think of three options:
(1) automatically publish the public key to the openpgp keyservers. this could pollute the keyservers. it also would create problems when we are testing, but this is not too bad i suppose. maybe we could create short lived keys.
(2) we could attach the user's public key to their outgoing email. it could be attached either to every email, or only to the first email sent to a new address (the latter is better, but requires we track all the addresses).
(3) we could set a default signature with some text like "I prefer encrypted email. My public key is available at https://bitmask.net/keys/elijah" or something.
(from redmine: created on 2013-10-23, closed on 2013-12-05)