database: allow for aliases to have a public key

we have email aliases, which is cool, but now that i think about it, what we really need is for each aliases to also have its own public key. otherwise, aliases will provide much less security than the main address.

eventually, what is needed is an API for adding aliases and uploading keys. for now, i just want to make sure that the database format that things are stored in is set up to be compatible with this future need.

we don't have a live email database anywhere yet, so now is the time to modify the format of the email db storage structure.

(from redmine: created on 2013-07-01, closed on 2013-09-06)