### What changes were proposed in this pull request?
Spark's Web UI is using an older version of Bootstrap (v. 2.3.2) for the portal pages. Bootstrap 2.x was moved to EOL in Aug 2013 and Bootstrap 3.x was moved to EOL in July 2019 (https://github.com/twbs/release). Older versions of Bootstrap are also getting flagged in security scans for various CVEs:
https://snyk.io/vuln/SNYK-JS-BOOTSTRAP-72889https://snyk.io/vuln/SNYK-JS-BOOTSTRAP-173700https://snyk.io/vuln/npm:bootstrap:20180529https://snyk.io/vuln/npm:bootstrap:20160627
I haven't validated each CVE, but it would be nice to resolve any potential issues and get on a supported release.
The bad news is that there have been quite a few changes between Bootstrap 2 and Bootstrap 4. I've tried updating the library, refactoring/tweaking the CSS and JS to maintain a similar appearance and functionality, and testing the UI for functionality and appearance. This is a fairly large change so I'm sure additional testing and fixes will be needed.
### How was this patch tested?
This has been manually tested, but there is a ton of functionality and there are many pages and detail pages so it is very possible bugs introduced from the upgrade were missed. Additional testing and feedback is welcomed. If it appears a whole page was missed let me know and I'll take a pass at addressing that page/section.
Closes#27370 from clarkead/bootstrap4-core-upgrade.
Authored-by: Dale Clarke <a.dale.clarke@gmail.com>
Signed-off-by: Gengliang Wang <gengliang.wang@databricks.com>
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