fix: Disable lazy loading for the Database selector

[michael-s-molina]

SUMMARY

Disables the lazy loading for the Database selector. Currently, the SQL Editor depends on some information that is only present when querying the databases like allows_virtual_table_explore. Since this information is not present anywhere, we need to force the query of the database selector when first loading the page.

@jinghua-qa

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Screen.Recording.2021-09-29.at.4.32.08.PM.mov

Screen.Recording.2021-09-29.at.4.29.34.PM.mov

TESTING INSTRUCTIONS

Check the videos for 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

[geido]

LGTM. The tests need some love

[codecov]

Codecov Report

Merging #16908 (fd08cf2) into master (4da4fe4) will increase coverage by 0.01%.
The diff coverage is 33.33%.

❗ Current head fd08cf2 differs from pull request most recent head 5ac3781. Consider uploading reports for the commit 5ac3781 to get more accurate results

@@            Coverage Diff             @@
##           master   #16908      +/-   ##
==========================================
+ Coverage   77.02%   77.04%   +0.01%     
==========================================
  Files        1022     1021       -1     
  Lines       54857    54846      -11     
  Branches     7482     7481       -1     
==========================================
- Hits        42256    42255       -1     
+ Misses      12354    12345       -9     
+ Partials      247      246       -1     

Flag	Coverage Δ
javascript	71.16% <20.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
...frontend/src/components/DatabaseSelector/index.tsx	95.00% <ø> (+2.50%)	⬆️
superset-frontend/src/views/CRUD/hooks.ts	47.23% <ø> (ø)
superset/datasets/commands/update.py	94.11% <ø> (ø)
superset/datasets/dao.py	96.50% <ø> (-0.12%)	⬇️
...erset-frontend/src/SqlLab/components/ResultSet.tsx	61.94% <20.00%> (-1.42%)	⬇️
superset/databases/api.py	93.00% <100.00%> (+0.04%)	⬆️
superset-frontend/src/SqlLab/actions/sqlLab.js	58.69% <0.00%> (+0.40%)	⬆️
...rontend/src/SqlLab/components/SqlEditorLeftBar.jsx	58.20% <0.00%> (+2.98%)	⬆️

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