Move Global scss variable into `global-styles/variables.scss`

Created by: gitstart-sourcegraph

Description

It's confusing to see global CSS variables related to input in multiple places.

Implementation Details

Move all global CSS variables affected by this PR into a global-styles/variables.scss and import it in global-styles/index.scss

Refs

• Sourcegraph Issue
• Gitstart Ticket

Preview App

• Frontend
• Storybook

Test plan

• Compare UI on local and remote SourceGraph app
• Confirm that there is no diff in both versions