Run BillingDocumentAdmin.download_selected_documents asynchronous

[zalmanu]

The generation takes a lot of time and the user can get nginx (and gunicorn) timeout.

[presslabs-bot]

This issue will be automatically closed on Tue, 17 Jan 2017 due to the lack of activity (comments or updates). Closed issues that remain inactive for a long period of time might also get automatically locked.

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cc: @calind, @pdobrescu, @vtemian