[SQL Lab] Re-add erroneously removed css

[etr2460]

CATEGORY

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• Bug Fix
• Enhancement (new features, refinement)
• Refactor
• Add tests
• Build / Development Environment
• Documentation

SUMMARY

#8563 removed some assumed dead css, but it's actually still being used to style the gripper for resizing the sql editor and closing the side pane. This PR readds it.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before:

After:

TEST PLAN

ensure you can resize the sql editor and open and close the side pane

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

REVIEWERS

@rusackas @graceguo-supercat @nytai

[codecov-io]

Codecov Report

Merging #8707 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8707      +/-   ##
==========================================
+ Coverage   65.51%   65.55%   +0.04%     
==========================================
  Files         481      481              
  Lines       24081    24081              
  Branches     2763     2763              
==========================================
+ Hits        15776    15786      +10     
+ Misses       8125     8115      -10     
  Partials      180      180

Impacted Files	Coverage Δ
superset/db_engine_specs/postgres.py	83.33% <0%> (+27.77%)	⬆️

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[rusackas]

Wish I could approve this for real, but I'll give it the gray checkmark at least... and thanks for spotting/fixing this. Apologies for the heavy-handed deletion.