Import translations from Bazaar branches hosted in Launchpad

Extend template import from branches with import with support for PO and XPI files containing translations. UI options for productseries would have to be extended to support selecting this (for "just a template", we should do a one-time import of translations as well, but should allow regular imports of translations as a separate option), and scanner should pick up PO/XPI files and put those into queue as well.