Give the resource classes api_url properties

[jwodder]

I was getting annoyed by having to type out API URLs the long way.

This PR also fixes the URL parsing to accept API URLs with trailing slashes and adds tests to ensure that all API URL formats are accepted with & without trailing slashes.

[codecov]

Codecov Report

Merging #816 (319d27e) into master (3fcc02d) will increase coverage by 0.03%.
The diff coverage is 97.61%.

@@            Coverage Diff             @@
##           master     #816      +/-   ##
==========================================
+ Coverage   84.69%   84.72%   +0.03%     
==========================================
  Files          61       61              
  Lines        6188     6201      +13     
==========================================
+ Hits         5241     5254      +13     
  Misses        947      947              

Flag	Coverage Δ
unittests	84.72% <97.61%> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
dandi/dandiarchive.py	83.83% <ø> (ø)
dandi/tests/test_dandiarchive.py	100.00% <ø> (ø)
dandi/dandiapi.py	86.59% <91.66%> (+0.28%)	⬆️
dandi/tests/test_dandiapi.py	100.00% <100.00%> (ø)
dandi/tests/test_delete.py	100.00% <100.00%> (ø)
dandi/tests/test_download.py	100.00% <100.00%> (ø)
dandi/tests/test_upload.py	100.00% <100.00%> (ø)

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[yarikoptic]

Looks good, thanks!