Add subresource integrity to script tag

[d9pouces]

Allow to integrate subresource-integrity attributes to <script tags>.
Require to add the two following keys beside "output_filename" in Django's setting PIPELINE['JAVASCRIPT']["your package"]:

"crossorigin": "anonymous",
"integrity": "sha384",

Of course, "sha256" and "sha512" also works.
Hashes are computed at runtime and cached to minimize code changes.

Cf. https://infosec.mozilla.org/guidelines/web_security#subresource-integrity

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (14b6d2c) 82.35% compared to head (463a415) 82.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #805      +/-   ##
==========================================
+ Coverage   82.35%   82.68%   +0.33%     
==========================================
  Files          33       33              
  Lines        1139     1155      +16     
==========================================
+ Hits          938      955      +17     
+ Misses        201      200       -1     

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