Extract bootstrap style import `@import 'bootstrap/scss/variables'`

Created by: gitstart-sourcegraph

Problem statement

We depend on Bootstrap styles in our global styles a lot. But we don't plan to move forward with Bootstrap because we're working on our design system called Wildcard. To ease the transition to our styles and gain more control over visual changes, we want to move Bootstrap styles that we need into our codebase and eventually drop Bootstrap dependency.

Ref

https://github.com/sourcegraph/sourcegraph/issues/29200

Success criteria

1. All highlighted imports has been removed
2. All css rules currently used from the imports are now available in style sheets

Implementation details

1. Remove bootstrap import @import 'bootstrap/scss/variables
2. Verify variables that are still used and not overridden on app
3. Copy them into internal variable file client/branded/src/global-styles/variables.scss

Time estimate

• Pull requests with frontend lines changed should take 3 hours at maximum. Ping the reviewer in the spec pull request if time-consuming changes are required.
• Split the work into multiple pull requests if the total diff is bigger than 450 lines of code.