Move BaseSeriesTemplatesView query into TranslationTemplatesCollection

The complex query in the iter_data method of BaseSeriesTemplatesView
should be moved into HasTranslationTemplatesMixin to allow one to fetch
this data (eg. getTemplatesWithSharingData) and at least extend the
TranslationTemplatesCollection itself to have things like
"joinOtherSidePOTemplate" which would include basically the logic
contained in the view today (except that it would work both from the
distroseries and from the productseries side)

See https://code.launchpad.net/~benji/launchpad/bug-734765/+merge/69554 for
a related branch and discussion.