wip: add nginx mod_zip output feature

[faudebert]

This a draft pull-request (i.e. no test, no doc yet). I would be happy to get feedback to know if maintainers are interested by such a feature. If so, I will take time to make it cleaner including docs, samples and tests.

It's great being able to provide an album download link thanks to zip_gallery plugin. Nonetheless, on my Sigal instance, it is a seldomly used feature. Thus, having many zip archives on my hard drive which duplicates all my original pictures is a waste of disk storage.

The idea of this pull-request is to make Sigal generate NGINX mod_zip "manifest" instead of real zip archives. Then, on user request, a properly configured NGINX will take care of dynamically generating the zip archive using the "manifest".

P.S.: I've currently re-used zip_gallery since it's very similar job (only the output result change), not sure it's the best approach though.

[codecov]

Codecov Report

Merging #391 into master will decrease coverage by 0.97%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master     #391      +/-   ##
==========================================
- Coverage   86.28%   85.31%   -0.98%     
==========================================
  Files          22       22              
  Lines        1750     1777      +27     
==========================================
+ Hits         1510     1516       +6     
- Misses        240      261      +21     

Impacted Files	Coverage Δ
sigal/plugins/zip_gallery.py	63.23% <25.00%> (-27.01%)	⬇️
sigal/image.py	89.81% <0.00%> (ø)
sigal/plugins/feeds.py	97.05% <0.00%> (ø)
sigal/plugins/adjust.py	100.00% <0.00%> (ø)
sigal/plugins/upload_s3.py	0.00% <0.00%> (ø)
sigal/plugins/encrypt/encrypt.py	80.24% <0.00%> (ø)

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[saimn]

Seems very interesting ! 👍
Reusing zip_gallery seems to make sense.