PO export with Last-Translator " <Unknown>" email address

I think this is hard to decide on. I don't think either way is "correct", and is mostly a matter of preference. We do not, in general, offer those PO files for download, and maybe we should just put a LP user name in there (i.e. something like https://launchpad.net/~user), but that would break every software handling Last-Translator field.

Of course, dealing with imports is going to be a nice excercise, and this would require a lot of effort, for somewhat dubious gain (i.e. I am not sure it's the right thing to do, even if someone asks for it to be done; what we are definitely doing wrong is not allowing a person to choose their email address that should be in such PO files, but that would be as hard, if not harder to implement).

Also affects bzr imports and exports. I.e. we've gotten reports on monajat (https://translations.edge.launchpad.net/monajat/trunk/+translations), with their bzr export and import (i.e. see how we export "Unknown" Last-Translator, then re-import it and lose the credits in rev 50 of lp:~sabily.team/monajat/translations).

The good way to fix this would be to use the Launchpad user homepage link (i.e. https://launchpad.net/~username) in the Last-Translator instead of an email and then parse that on import (including full URL so it's more specific than just "username").

To change what's exported, look at lib/lp/translations/model/pofile.py POFileToTranslationFileDataAdapter.header property (search for email = 'Unknown'). One would get the full user path using canonical_url(self._pofile.lasttranslator), and then they'd have to parse that in POHeader.getLastTranslator() in lib/lp/translations/utilities/gettext_po_parser.py. Appropriate unit tests would be needed as well, but one can read about our processes up on https://dev.launchpad.net/ or ask on #launchpad-dev on FreeNode IRC.

Rather than ~username, which is mutable, perhaps we could use some persistent id (I think we have one for openid).