Add --only-metadata option to migrate-dandisets.py

[jwodder]

Closes #510.

[codecov]

Codecov Report

Merging #511 (c67f29e) into master (02b2037) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #511   +/-   ##
=======================================
  Coverage   82.64%   82.64%           
=======================================
  Files          59       59           
  Lines        6107     6107           
=======================================
  Hits         5047     5047           
  Misses       1060     1060           

Flag	Coverage Δ
unittests	82.64% <ø> (ø)

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