remove bootstrap style import `@import 'bootstrap/scss/custom-forms`

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

let's also remove the following imports and copy CSS rules that we need to our CSS files:

1. remove the import @import 'bootstrap/scss/custom-forms from global index.scss file
2. copy styles used in codebase into new global custom-forms.scss file within client/branded/src/global-styles
3. import file into global index.scss

Note: some style rules from @import 'bootstrap/scss/custom-forms are already being modified by styles in forms.scss

Time estimate

• Pull requests with ~450 lines changed should take 5 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.