fix(dashboard): allow users to resize the markdown widget easier

[cccs-Dustin]

SUMMARY

This PR was made to fix the issue where users were not easily able to resize the column width of the Markdown widget when editing a dashboard. From my investigation of the code, I found that there was indeed a container handle on the right side of a Markdown widget, it was just very narrow compared to the right side container handles of the other charts within a dashboard.

To fix this issue, I just needed to modify the resizable.less file so that .resizable-container-handle--right behaves the same way as .resizable-container-handle--bottom.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before:
(The resize cursor is not easily selectable, as the handle to resize it is very narrow)

After:
(The resize cursor is now much easier to select, as the handle for it has been widened and is now the same size as the handle on the bottom of the widget)

TESTING INSTRUCTIONS

1. Go to a dashboard which contains a Markdown widget, or add a Markdown widget to a new or existing dashboard.
2. Select the "Edit dashboard" button (pencil icon in the top right of the page).
3. Try to adjust the width of the markdown widget.
4. You will notice that it is now much easier to select the resize handle compared to before the code changes.

ADDITIONAL INFORMATION

• Has associated issue: The Column Resize Selection Area Is Too Narrow On The Markdown Dashboard Widget  #19838
• 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 #19839 (b643225) into master (a1bd5b2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head b643225 differs from pull request most recent head b86c34e. Consider uploading reports for the commit b86c34e to get more accurate results

@@           Coverage Diff           @@
##           master   #19839   +/-   ##
=======================================
  Coverage   66.55%   66.55%           
=======================================
  Files        1692     1692           
  Lines       64801    64802    +1     
  Branches     6657     6657           
=======================================
+ Hits        43128    43129    +1     
  Misses      19973    19973           
  Partials     1700     1700           

Flag	Coverage Δ
javascript	51.25% <ø> (ø)

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Impacted Files	Coverage Δ
superset-frontend/src/SqlLab/actions/sqlLab.js	60.39% <ø> (ø)
superset/utils/csv.py	97.67% <100.00%> (+0.05%)	⬆️

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[etr2460]

this seems reasonable, assuming you've tested this for all charts/dashboard components (and not only markdown)

[cccs-Dustin]

this seems reasonable, assuming you've tested this for all charts/dashboard components (and not only markdown)

I tested with all of the charts/dashboard components. It seems to work correctly for all of them now, not just everything but the markdown component.