[Security] Bump bootstrap from 4.1.3 to 4.3.1

[dependabot-preview]

Bumps bootstrap from 4.1.3 to 4.3.1.

Release notes

Sourced from bootstrap's releases.

v4.2.1

• Bootstrap rubygem now depends on SassC instead of Sass.

Changelog

Sourced from bootstrap's changelog.

Changelog

The changelog is tracked here but also in the Releases section of the GitHub project.
The changelog only includes changes specific to the RubyGem.

The Bootstrap framework changes can be found in the Releases section of twbs/bootstrap.
Release announcement posts on the official Bootstrap blog contain summaries of the most noteworthy changes made in each release of Bootstrap.

4.2.1

• Bootstrap rubygem now depends on SassC instead of Sass.

4.0.0.beta2.1

Fixes an extraneous sourceMappingURL in bootstrap.js.
#124

4.0.0.beta2

Compass is no longer supported. Minimum required Sass version is now v3.5.2.
#122

4.0.0.alpha3.1

This release corresponds to the upstream Bootstrap 4 Alpha 3.

Commits

• cbbed2d Bump bootstrap to 4.3.1
• a63d04c rake update[v4.3.1]
• abf9232 Updater: Exclude tools/sanitizer.js from bootstrap_js_files
• 0a29879 Bump to v4.3.0
• c406232 rake update[v4.3.0]
• 2f54168 Bump to v4.2.1
• e8dde96 rake update[v4.2.1]
• 4192f90 rake update[v4-dev]
• 804e3dd Depend on sassc-rails, specify required_ruby_version
• 2277fb3 Set min precision to 6 as per upstream
• Additional commits viewable in compare view

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[dependabot-preview]

We've just been alerted that this update fixes a security vulnerability:

Sourced from The GitHub Security Advisory Database.

Moderate severity vulnerability that affects bootstrap and bootstrap-sass
In Bootstrap 4 before 4.3.1 and Bootstrap 3 before 3.4.1, XSS is possible in the tooltip or popover data-template attribute. For more information, see: https://blog.getbootstrap.com/2019/02/13/bootstrap-4-3-1-and-3-4-1/

Affected versions: ["< 4.3.1"]

[mattbrictson]

@marclove we had been holding off on upgrading bootstrap until Rails 6 (see #150) because bootstrap 4.3.1 also forces us to switch to sassc. But I think we should pull the trigger now given this is now a security issue. I will do a quick smoke test before merging.

[mattbrictson]

Generated a scaffold and the bootstrap styling looks fine and I didn't encounter and errors or warnings. LGTM 🚀