Duplicated code in translations upload forms

The methods POTemplateView.upload(), POFileUploadView.process_form(), and ProductSeriesView.translationsUpload() are suspiciously similar. The POTemplateView and ProductSeriesView ones really only differ in the arguments to addOrUpdateEntry/addOrUpdateEntriesFromTarball, and how a link to the import queue is generated.

The POFileUploadView one is similar, but adds the choice between published and user uploads, and does not support tarballs. A mixin could probably provide all three.