Allow to integrate subresource-integrity attributes to javascript and…

[d9pouces]

… stylesheet tags.

Require to add the two following keys beside "output_filename" in Django's setting PIPELINE['JAVASCRIPT']["your package"] and PIPELINE['STYLESHEETS']["your package"] and :

"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

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.75%. Comparing base (179313f) to head (b117dc8).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
pipeline/packager.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #815      +/-   ##
==========================================
+ Coverage   82.50%   82.75%   +0.25%     
==========================================
  Files          33       33              
  Lines        1160     1177      +17     
==========================================
+ Hits          957      974      +17     
  Misses        203      203              

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