Fix content jump due to html scrollbar

What does this MR do?

Stop content jumping when opening modals, and hide those unsightly modal scrollbars

Remedies #21729 (closed)

Screenshots

before	after
Screen_Recording_2020-06-04_at_9.41.24_pm	Screen_Recording_2020-06-04_at_9.37.52_pm

How to enable always-on scrollbars on macos

Settings > General > Show scroll bars: Always

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Style guides

Availability and Testing