Better filtering of file-only log messages

[jwodder]

This PR modifies the way that logfile-only log messages are filtered out from the stderr logs to be more robust/standard/"the right way to do things" — specifically, by filtering on a custom extra log record attribute instead of the log message.

[codecov]

Codecov Report

Merging #523 (21f0a10) into master (0e63181) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #523   +/-   ##
=======================================
  Coverage   83.32%   83.32%           
=======================================
  Files          62       62           
  Lines        6382     6382           
=======================================
  Hits         5318     5318           
  Misses       1064     1064           

Flag	Coverage Δ
unittests	83.32% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dandi/cli/command.py	46.05% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0e63181...21f0a10. Read the comment docs.