360 photo and video support

[mittman]

Basic integration of Valiant360 webGL library to view 360-degree photos and videos within sigal gallery. Modification of galleria theme, click VR icon or on the active image to open modal.

[codecov]

Codecov Report

Merging #367 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #367   +/-   ##
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  Coverage   87.38%   87.38%           
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  Files          18       18           
  Lines        1443     1443           
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  Hits         1261     1261           
  Misses        182      182

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[saimn]

Hi,
Thanks for the PR, this seems to be a nice feature. However I'm not sure to want this in the core galleria theme, because this means more dependencies, and for each release it is a lot of work to update all the dependencies to their latest versions and check that everything still works.
What would think about putting this in another repository, like the 153957-theme (which can be pip-installed, which is great but not strictly necessary) ? The theme could then be referenced in https://github.com/saimn/sigal/wiki with other community efforts.