Introduce DapperScrollbars

[alexpaxton]

Closes #12903
Closes #12630
Closes #12629
Closes #12989

Changes

Introduce DapperScrollbars wrapper for react-scrollbars-custom package as a replacement for FancyScrollbars & react-custom-scrollbars

Implement DapperScrollbars in higher level components as well as areas that were problematic with FancyScrollbars

Style DapperScrollbars to mostly match the appearance of FancyScrollbars except for that the scrollbar track is slightly visible in DapperScrollbars (useful for discerning which scroll component is being used at a glance).

Preview

Gotchas

• Partially because the underlying library works a little differently, dropdowns with a menu that is wider than the button must now be explicitly passed a width rather than it being automatically determined. After reviewing the UI this only is needed in a few places.

Tech Debt:

• Replace FancyScrollbars with DapperScrollbars #12960 (Replace FancyScrollbars with DapperScrollbars so we're not supporting both)
• Sometimes the scrollbar flickers before disappearing on dropdown menus that don't have enough contents to scroll
• All the styles to customize the appearance of DapperScrollbars are relying on !important to override some of the inline styles injected by the library
  • The library has an option to not inject any inline styles, which allows for more customization, but it doesn't work as expected

Checklist:

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Rebased/mergeable
• Tests pass
• http/swagger.yml updated (if modified Go structs or API)
• Sign CLA (if not already signed)

[chnn]

Should this be PureComponent?

[AlirieGray]

Looks great!

[OfTheDelmer]

Would this help with any of these: #12989 #12987 #12986

[alexpaxton]

@OfTheDelmer it would close #12989