Allow project maintainers to approve translation imports

Translation import approval is currently done by the Launchpad team. We should instead allow project maintainers to do it themselves, and provide more of the exit strategies for when things go wrong (approve a template into the wrong template, remove/deactivate a template, unshare automatically).

This bug is the placeholder bug that will list all the relevant tasks as I investigate and figure them out.

 - Allow project maintainers TranslationAdmin privilege on the import queue for their project
 - Check where else is TranslationAdmin privilege used to ensure we are not widening the scope unintentionally
 - Check how the template splitting/merging currently works and see what it takes to make it work for all the templates
 - Ensure PO files use English as the base language (some heuristic)?
 - Removal of old translations so they don't pollute the translation memory
 - Write up user-friendly documentation for template queue management