Enable beautiful json for dashboard metadata editor

[ktmud]

Try #9474 in a new branch to trigger CI.

CATEGORY

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

SUMMARY

Reformat JSON strings in the dashboard metadata modal input so it's easier to edit.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before

After

TEST PLAN

1. Pick a random dashboard with filters
2. Edit metadata via "Edit dashboard" -> "" -> "Edit dashboard properties" -> "Advanced".

The JSON string in the code editor should be formatted.

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

@graceguo-supercat @etr2460

[nytai]

welp.. looks like the job was received: https://travis-ci.org/github/apache/incubator-superset/builds/671939629?utm_medium=notification&utm_source=github_status but it hasn't notified github

[codecov-io]

Codecov Report

Merging #9482 into master will decrease coverage by 0.01%.
The diff coverage is 20.00%.

@@            Coverage Diff             @@
##           master    #9482      +/-   ##
==========================================
- Coverage   58.76%   58.75%   -0.02%     
==========================================
  Files         385      385              
  Lines       12237    12241       +4     
  Branches     3020     3020              
==========================================
+ Hits         7191     7192       +1     
- Misses       4862     4865       +3     
  Partials      184      184              

Impacted Files	Coverage Δ
...ntend/src/dashboard/components/PropertiesModal.jsx	46.83% <20.00%> (-1.17%)	⬇️

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