Add script for migrating Dandiset metadata

[jwodder]

Closes #360.

[codecov]

Codecov Report

Merging #366 (ffe2fac) into master (fc56dfb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #366   +/-   ##
=======================================
  Coverage   82.06%   82.06%           
=======================================
  Files          55       55           
  Lines        5453     5453           
=======================================
  Hits         4475     4475           
  Misses        978      978           

Flag	Coverage Δ
unittests	82.06% <ø> (ø)

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[yarikoptic]

I will test it in the battlefield ;) Thank you @jwodder