Add "Incomplete" status to import queue

    * Overview: (dis)approving entries in the translation import queue (https://launchpad.net/translations/imports) sometimes involves a lot of manual work for entries which need review, so it may take a while for the response to be received -- sometimes, this falls between the cracks and we can forget where we need to wait for their reply
    * Goal/Deliverables: add new status value "Incomplete" (we looked at it and already contacted uploaders/maintainers to provide further information), and update rosetta-approve-imports.py script to "Delete" incomplete entries after 2 weeks (also rename the script to something more generic)
    * Full spec: None
    * Depends on: n/a

We will know we have finished when we have the new status field 'Incomplete' to be used when we wait for answer from the uploader, and which will turn into 'Deleted' if we don't receive a response after two weeks.