Make existing="refresh" a synonym for "overwrite" for new upload

[jwodder]

Closes #387.

[codecov]

Codecov Report

Merging #390 (2f52473) into master (ea86eee) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #390      +/-   ##
==========================================
+ Coverage   82.38%   82.54%   +0.15%     
==========================================
  Files          55       55              
  Lines        5705     5780      +75     
==========================================
+ Hits         4700     4771      +71     
- Misses       1005     1009       +4     

Flag	Coverage Δ
unittests	82.54% <100.00%> (+0.15%)	⬆️

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Impacted Files	Coverage Δ
dandi/tests/test_upload.py	91.27% <100.00%> (+0.05%)	⬆️
dandi/upload.py	77.21% <100.00%> (+1.15%)	⬆️
dandi/dandiapi.py	88.36% <0.00%> (+0.80%)	⬆️

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