Contributors should list only those which have contributed directly through LP

[Данило Шеган]

Contributors should generally only list translators who contributed directly through Launchpad. While it'd be useful to list upstream translators as well, they are usually credited in translator-credits and similar messages as well.

Reasons:
 * History should only show what you've done in Launchpad to make it easier to find stuff you're likely to work on
 * Translator credits should be *updated* with only Launchpad translators (fwiw, the title in the automatically generated message will say "Launchpad Contributions")

There is a lot of value in being able to see a list of imported contributors as well, so we might want to keep both in pofiletranslator, but easily distinguished.

A query to recalculate POFileTranslator to only contain LP ones:

 insert into pofiletranslator (person, pofile, latest_message) select distinct on (submitter,pofile.id) submitter, pofile.id, translationmessage.id from translationmessage join translationtemplateitem on translationmessage.potmsgset=translationtemplateitem.potmsgset join pofile on translationtemplateitem.potemplate=pofile.potemplate where pofile.language is not distinct from translationmessage.language and pofile.variant is not distinct from translationmessage.variant and translationmessage.is_imported is false;