chore(thumbnails): change default executor to logged in user

[villebro]

SUMMARY

This changes Thumbnails to execute as the currently logged in user by default, and fall back to the Selenium user for anonymous users (=when there is no logged in user). This option was introduced in #22328

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #22801 (6f4bcba) into master (5192925) will increase coverage by 1.60%.
The diff coverage is 100.00%.

❗ Current head 6f4bcba differs from pull request most recent head 6a69159. Consider uploading reports for the commit 6a69159 to get more accurate results

@@            Coverage Diff             @@
##           master   #22801      +/-   ##
==========================================
+ Coverage   66.70%   68.31%   +1.60%     
==========================================
  Files        1951     1951              
  Lines       75385    75385              
  Branches     8185     8185              
==========================================
+ Hits        50283    51496    +1213     
+ Misses      22999    21786    -1213     
  Partials     2103     2103              

Flag	Coverage Δ
hive	53.19% <100.00%> (ø)
mysql	78.96% <100.00%> (+<0.01%)	⬆️
postgres	79.03% <100.00%> (+<0.01%)	⬆️
presto	53.12% <100.00%> (ø)
python	82.83% <100.00%> (+3.32%)	⬆️
sqlite	77.56% <100.00%> (+<0.01%)	⬆️
unit	53.11% <100.00%> (?)

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

Impacted Files	Coverage Δ
superset/config.py	92.02% <100.00%> (ø)

... and 89 files with indirect coverage changes

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[michael-s-molina]

LGTM