oonitemplates: compute body simhash and sha256sum

[bassosimone]

No description provided.

[hellais]

relevant: https://github.com/ooni/backend/issues/105

[bassosimone]

In #1727 I run some experiments using TrendMicro's tlsh to measure pages equality. I see an hash such as tlsh useful to state whether the page returned by the control is the same as the page observed by the probe. On the contrary, simhash is useful to measure the distance but it's still unclear to me exactly how we can reduce that into the space of equality.

[bassosimone]

Duplicates #1719