chore(welcome): harmonize submenu with other routes

[villebro]

SUMMARY

The welcome page submenu currently does not use the same SubMenu component that's used on the other routes, like Dashboard, Chart, Dataset etc, causing it to deviate from the normal look and feel. This fixes that. At the same time I noticed that we were using a re-export of FeatureFlag from @superset-ui/core in lots of places, which I believe is a remnant from the multirepo days. This removes the export and replaces all occurences with a direct import from @superset-ui/core.

AFTER

Now the Welcome page looks the same as the other pages:

BEFORE

Previously the "Home" text had a different font size, and the text next to the toggle was not using the proper theme.

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 #22803 (614449f) into master (0fa4212) will increase coverage by 0.00%.
The diff coverage is 85.00%.

❗ Current head 614449f differs from pull request most recent head 640a45d. Consider uploading reports for the commit 640a45d to get more accurate results

@@           Coverage Diff           @@
##           master   #22803   +/-   ##
=======================================
  Coverage   67.65%   67.65%           
=======================================
  Files        1909     1910    +1     
  Lines       73743    73749    +6     
  Branches     7987     7987           
=======================================
+ Hits        49888    49894    +6     
  Misses      21814    21814           
  Partials     2041     2041           

Flag	Coverage Δ
javascript	53.85% <85.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset-frontend/src/SqlLab/App.jsx	0.00% <ø> (ø)
superset-frontend/src/SqlLab/actions/sqlLab.js	64.50% <ø> (ø)
...d/src/SqlLab/components/ShareSqlLabQuery/index.tsx	81.48% <ø> (ø)
...frontend/src/SqlLab/components/SouthPane/index.tsx	79.54% <ø> (ø)
...frontend/src/SqlLab/components/SqlEditor/index.jsx	56.68% <ø> (ø)
...d/src/SqlLab/components/TabbedSqlEditors/index.jsx	49.47% <ø> (ø)
superset-frontend/src/components/Chart/Chart.jsx	55.35% <ø> (ø)
...erset-frontend/src/components/Chart/chartAction.js	55.34% <ø> (ø)
...end/src/components/Datasource/DatasourceEditor.jsx	65.35% <ø> (ø)
...tend/src/components/Datasource/DatasourceModal.tsx	69.76% <ø> (ø)
... and 40 more

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[michael-s-molina]

LGTM. Thanks for the awesome improvements!