Reuse images: change ImageRun keys to be based on image data content

[mustache1up]

In order to generate only one media file for repeated images with the same data the uniqueId of the image can be based on the image data SHA1 digest (same strategy git uses internally).

Fixes #2680 and #849

[dolanmiu]

This seems like a good idea

But the dependency doesn't seem to be maintained?

https://github.com/indutny/hash.js

Edit: The library seems promising somewhat, as it works for both Node and Browser

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (048a035) to head (041e9cb).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2681   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          281       281           
  Lines        14091     14097    +6     
  Branches      1379      1382    +3     
=========================================
+ Hits         14091     14097    +6     

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