Import Queue Mass Status change

    * Overview: We need to be able to set single status on several entries in the general translation import queue at once -- this is mostly preparation for another spec, and that's providing template emails for status changes.
    * Goal/Deliverables: Combo boxes for status are to be replaced with checkboxes for each entry, and a global status combo box for changing status for all entries.
    * Full spec: https://launchpad.canonical.com/Translations/ImprovingImportQueueManagement
    * Depends on: n/a

We will know we have finished when we can change the status for many entries in the translation import queue with a few clicks.