CVE-2017-7481
Lookup returns wrap the result in unsafe, however when used through the
standard templar engine, this does not result in the jinja2 environment being
marked as unsafe as a whole. This means the lookup result looses the unsafe
protection and may become simple unicode strings, which can result in bad
things being re-templated.
This also adds a global lookup param and cfg options for lookups to allow
unsafe returns, so users can force the previous (insecure) behavior.
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