Remove style duplication caused by the `forkedBranded.scss` file

Created by: valerybugakov

Problem statement

Duplicated branded.scss in the VSCode package makes it hard to keep the theme consistent between all Sourcegraph clients.

Success criteria

The forkedBranded.scss file is removed in favor of the branded.scss import.

Implementation details

1. Rename client/branded/src/global-styles/index.scss to client/branded/src/global-styles/base.scss
2. Move @import './highlight'; from base.scss to client/branded/src/global-styles/index.scss
3. client/vscode/src/webview/index.scss should import client/branded/src/global-styles/base.scss instead of forkedBranded.scss
4. All style changes specific to the VSCode theme should be made in the client/vscode/src/webview/index.scss

Time estimate

• The task should take 8 hours at maximum. Ping the reviewer in the spec pull request if time-consuming changes are required.