[TMP] Fix Sass dependencies import for Bower

⚠️ **This is a temporary solution**

Unlike NPM, Bower does not have its own registry and uses the Github
repository. Like `js, `sass` and `dist`, all the files required by
Bower must be published on Git.

Changes:
- Pin the Sass dependencies versions and update `yarn.lock`
- Move only the require files to `/vendor`
- Publish the Sass dependencies in `vendor/` on git.