chore: add SAMPLES_ROW_LIMIT in frontend conf

[zhaoyongjie]

SUMMARY

Let the frontend knows what is SAMPLES_ROW_LIMIT value.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

The redux of explore or dashboard should access this config item.

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 #20841 (c52192d) into master (9bf7ed5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #20841   +/-   ##
=======================================
  Coverage   66.33%   66.33%           
=======================================
  Files        1756     1756           
  Lines       66769    66769           
  Branches     7059     7059           
=======================================
  Hits        44288    44288           
  Misses      20684    20684           
  Partials     1797     1797           

Flag	Coverage Δ
hive	53.25% <ø> (ø)
mysql	81.06% <ø> (ø)
postgres	81.13% <ø> (ø)
presto	53.11% <ø> (ø)
python	81.57% <ø> (ø)
sqlite	79.74% <ø> (ø)
unit	50.25% <ø> (ø)

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

Impacted Files	Coverage Δ
superset/views/base.py	75.36% <ø> (ø)

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[stephenLYZ]

LGTM!