Only consider webapp categories in App API (bug 963129)

I initially removed that because specifying the queryset did an extra
useless query, but it turns out it's not longer the case.
Merge pull request #1759 from diox/fix-api-webapp-categories

Only consider webapp categories in App API (bug 963129)
Converted legal docs to HTML.  Today's lucky number is 18622.
revert django-1.6-dependent code committed too soon
Rename webpay_tasks to tasks (bug 967786)

- Allows celery to auto discover tasks and register with the default queue
Merge pull request #1762 from JaredKerim-Mozilla/967786

Register send_payment_receipt task with celery (bug 967786)
Updated compile-mo.sh to do .po linting
adding Bengali
Updated compile-mo.sh to activate venv for deploy
add Bengali to Marketplace
Fix tests