Revert "Drop docs for unsupported OSCAR_ALLOW_ANON_REVIEWS setting"

This reverts commit 7075ba659409cee5f76c3a611d211344c86b46f0.
Add Product.is_review_permitted check

Default implementation respects OSCAR_ALLOW_ANON_REVIEWS and only allows
one review per user. Should be easy to customise (e.g. enforcing the
user purchased the product).
Switch to is_review_permitted in templates

Also removes the now obsolete has_review_by template filter.
Update release notes to include OSCAR_ALLOW_ANON_REVIEWS
Merge pull request #1000 from tangentlabs/issue/1000/anonreviews

Add functionality to disallow anonymous reviews