Extract bootstrap style import `@import 'bootstrap/scss/function'`, `@import 'bootstrap/scss/mixins'`

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 bootstrap's variables and mixins currently used from the imports are now available in style sheets.

Implementation details

1. Remove bootstrap import @import 'bootstrap/scss/mixins
2. Remove bootstrap import @import 'bootstrap/scss/functions
3. Verify that we still have any usage of bootstrap's functions/mixins in app.
4. If there are any usage, migrate the internally.

Time estimate

• Pull requests with ~450 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.