'untranslated' filter slow with message sharing

Postgres chooses a bad query plan with existing 'untranslated' filter.

A fix means several things:
 * replace "POTMsgSet.id IN" with "TranslationTemplateItem.potmsgset IN"
 * add TranslationTemplateItem.potmsgset = TranslationTemplateItem.potmsgset to the translated query so index on that table is preferred (yeah, this looks silly, but improves the speed by more than 10x on staging DB).

Basically, the second change makes Postgres use translationtemplateitem__potemplate__potmsgset__key index instead of using POTMsgSet id index.

These two changes combined give us around 40x improvement, so on our biggest POTemplate (i.e. ddtp-universe package descriptions, with 45000 messages) we get queries in 550ms where we had 21s.