feat(dashboard): Display a loading spinner while dashboard is being saved

[kgabryje]

SUMMARY

On slower networks, saving the dashboard can take significant amount of time. During saving, there is no indicator that saving is actually happening, which might be confusing for a user.
This PR adds a loading spinner in the middle of the screen which appears after user clicks "Save" and disappears after saving action ends (with a success or failure).

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Screen.Recording.2023-01-04.at.18.15.25.mov

TESTING INSTRUCTIONS

1. Edit a dashboard
2. Go to browser dev tools and slow down your network (in Chrome Devtools -> Network tab -> Set throttling to "Slow 3g")
3. Click Save
4. Verify that the loading spinner is displayed
5. Verify that the loading spinner is removed when saving is done

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 #22588 (c32d871) into master (cf156f1) will increase coverage by 0.00%.
The diff coverage is 76.92%.

@@           Coverage Diff           @@
##           master   #22588   +/-   ##
=======================================
  Coverage   67.00%   67.00%           
=======================================
  Files        1859     1859           
  Lines       71019    71032   +13     
  Branches     7766     7767    +1     
=======================================
+ Hits        47587    47597   +10     
- Misses      21410    21413    +3     
  Partials     2022     2022           

Flag	Coverage Δ
javascript	53.86% <76.92%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset-frontend/src/dashboard/actions/hydrate.js	1.78% <ø> (ø)
...-frontend/src/dashboard/reducers/dashboardState.js	71.42% <0.00%> (-3.04%)	⬇️
...t-frontend/src/dashboard/actions/dashboardState.js	53.77% <87.50%> (+1.32%)	⬆️
...d/components/DashboardBuilder/DashboardBuilder.tsx	74.31% <100.00%> (+0.72%)	⬆️
...tend/plugins/legacy-plugin-chart-rose/src/index.js	66.66% <0.00%> (ø)
...nd/plugins/plugin-chart-echarts/src/Gauge/index.ts	50.00% <0.00%> (ø)
...nd/plugins/plugin-chart-echarts/src/Graph/index.ts	50.00% <0.00%> (ø)
...nd/plugins/plugin-chart-echarts/src/Radar/index.ts	50.00% <0.00%> (ø)
...d/plugins/plugin-chart-echarts/src/Funnel/index.ts	50.00% <0.00%> (ø)
...lugins/legacy-plugin-chart-event-flow/src/index.ts	50.00% <0.00%> (ø)
... and 4 more

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[villebro]

LGTM, great improvement!