Rename DandiMeta to DandisetMeta

[jwodder]

Closes #453.

The test failures are due to changes caused by dandi/dandi-archive#144. The fix for them is in #452. After that PR is merged, I will rebase this PR and then un-draft it.

After merging this PR, merge dandi/dandi-api-datasets#20.

[codecov]

Codecov Report

Merging #454 (b714908) into master (cbdd48f) will increase coverage by 0.05%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##           master     #454      +/-   ##
==========================================
+ Coverage   82.24%   82.29%   +0.05%     
==========================================
  Files          55       55              
  Lines        5744     5744              
==========================================
+ Hits         4724     4727       +3     
+ Misses       1020     1017       -3     

Flag	Coverage Δ
unittests	82.29% <70.00%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
dandi/cli/cmd_ls.py	60.73% <0.00%> (ø)
dandi/validate.py	59.42% <0.00%> (ø)
dandi/metadata.py	77.44% <100.00%> (ø)
dandi/models.py	86.19% <100.00%> (ø)
dandi/tests/test_metadata.py	100.00% <100.00%> (ø)
dandi/tests/test_models.py	100.00% <100.00%> (ø)
dandi/upload.py	76.38% <0.00%> (+0.72%)	⬆️

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[yarikoptic]

Thank you! Let's proceed