codemod: Migrate remaining `.scss` files to `.module.scss`

Created by: gitstart-sourcegraph

Problem Statement

We're in the process of migration from global CSS to CSS modules. To automate part of the process, the codemod has been created that can do most of the changes required automatically. It was partially applied to the codebase. As part of this issue, we need to apply the codemod to the rest of the codebase where significant manual work (similar to #21368) is not required.

Ref

Gitstart Ticket

Implementation details

1. client/shared/src/actions/ActionsNavItems.scss — let's move these styles to client/shared/src/actions/ActionsNavItems.module.scss into .action-item.
2. client/web/src/repo/docs/DocumentationIcon.scss — let's convert it to CSS module.
3. client/web/src/repo/releases/RepositoryReleasesArea.scss — let's delete it. It seems that it's not used anywhere.
4. client/web/src/repo/stats/RepositoryStatsPage.scss — classes from here are used in client/web/src/repo/stats/RepositoryStatsContributorsPage.tsx. Let's move them to the corresponding CSS module and delete redundant rules if there're any.
5. client/web/src/search/home/homepage.scss— let's move the CSS variables defined here to client/web/src/SourcegraphWebApp.scss.

Time estimate

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