Ensure that the webkit scrollbars are applied to About modal only

[AparnaKarve]

Fixes #4541

/cc @epwinchell

[miq-bot]

Checked commit AparnaKarve@e379a5b with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
1 file checked, 1 offense detected

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• 💣 💥 🔥 🚒 - Linter/Haml - Linter::Haml STDERR:

warning: parser/current is loading parser/ruby23, which recognizes
warning: 2.3.6-compliant syntax, but you are running 2.3.3.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.

[epwinchell]

@Hyperkid123 Please test. I will clean up the css when I'm able to do a styling PR for the modal.

[Hyperkid123]

Menu and page scrollbars are fixed.

[epwinchell]

@miq-bot add_reviewer @epwinchell

[epwinchell]

@miq-bot rm_label wip