Comment 13 for bug 128324

What needs doing to fix this bug:
 ∙ Providing a pre-defined 'is_current' but not 'is_imported' translation for all translation credits messages when new POFile is created in the DB. It can be the string of the form 'translation credits placeholder; is not used'.
 ∙ When a new POTMsgSet is added by a template import which is one of translation credit messages, we should provide the same placeholder translation for all existing PO files.
 ∙ Populate all translation-credits messages for existing PO files (to fix bad data already in DB).