Add .h264 as a supported video file extension

[rly]

Fix #1328

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (8a06a6e) 88.85% compared to head (b4fab27) 88.85%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1329   +/-   ##
=======================================
  Coverage   88.85%   88.85%           
=======================================
  Files          76       76           
  Lines       10200    10200           
=======================================
  Hits         9063     9063           
  Misses       1137     1137           

Flag	Coverage Δ
unittests	88.85% <100.00%> (ø)

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Files Changed	Coverage Δ
dandi/consts.py	98.11% <100.00%> (ø)

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[yarikoptic]

Sorry @rly for taking so long to get back to this one. In the light of @satra 's feedback and your follow up on #1328 (comment) - I will just close it for now. If you think we should resurrect -- let me know. Personally -- I think I never saw/used .h264 file, and indeed conversion into a more common/supported container might be of benefit to the end users.