Distinguish between pre- and post-validation when uploading

[jwodder]

This PR changes the names of the "validation" statuses when uploading to "pre-validation" (done client-side before uploading) and "post-validation" (done server-side after uploading) so that one can more accurately tell what state an upload is in.

[codecov]

Codecov Report

Merging #420 (71619d3) into master (fc5de49) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #420   +/-   ##
=======================================
  Coverage   82.25%   82.25%           
=======================================
  Files          55       55           
  Lines        5679     5679           
=======================================
  Hits         4671     4671           
  Misses       1008     1008           

Flag	Coverage Δ
unittests	82.25% <100.00%> (ø)

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Impacted Files	Coverage Δ
dandi/dandiapi.py	87.28% <100.00%> (ø)
dandi/upload.py	75.93% <100.00%> (ø)

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[yarikoptic]

I even wonder if we should make it even more to the point

• metadata check
• upload check

Using check instead of validation since shorter. Wdyt?

[jwodder]

@yarikoptic I feel like "upload check" doesn't really convey much meaning. Also, "metadata check" is the same length as "pre-validation".

[yarikoptic]

I meant that "metadata check" would be shorter than "metadata validation". But ok -- let's proceed with pre/post.