* new 'allow_offsite' parameter in OffsiteMiddleware
* document deprecated dont_filter flag in OffsiteMiddleware
* avoid deprecating dont_filter in OffsiteMiddleware
* Copy the code to the downloader mw.
* Add tests for allow_offsite in the downloader mw.
* Mark allow_offsite with reqmeta.
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Co-authored-by: Andrey Rakhmatullin <wrar@wrar.name>