[IMP] web: upgrade the cache to use SHA2 over SHA1

SHA-1 is a cryptographic hash function that have weaknesses known since
2005, it has been deprecated by the NIST [1] about 10 years ago in 2011
and Google [2] have been able to perform a collision attack in 2017.

We use SHA-1 in order to generate unique URL for resources that can be
cached by the browser: assets bundle, translations, qweb templates and
qweb images.

Although practical attacks still requires quite a lot of computational
resources, it is time to upgrade SHA-1 to SHA-2.

We have selected the SHA-512/256 variant of the SHA-2 algorithm as
replacement for SHA-1 for the following reasons:

* On 64 bits platform, SHA-512 is the fastest SHA-2 variant, it is only
  ~1.5x slower than SHA-1. [3]
* Keeping only the 256 foremost bits protects against both collision
  attacks and length extension attacks.
* The hexadecimal digest is only 24 chars longer than SHA-1 which is
  nice to have somewhat short URLs.

We have not used SHA-3 because:

* At the moment of writing, it is too slow (~3x slower than SHA-1) [3]
* It is not guaranteed to be available with the Python 3.5 `hashlib`
  module.
* One of the author of SHA-3 is Belgian.

[1] https://csrc.nist.gov/projects/hash-functions/nist-policy-on-hash-functions
[2] https://shattered.io/
[3] http://bench.cr.yp.to/results-hash.html
[4] http://www.commitstrip.com/en/2017/02/27/the-sha-1-alternative/

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