Bump webpack-dev-server from 2.11.5 to 3.1.11 in /knockio-frontend

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 2.11.5 to 3.1.11.
- [Release notes](https://github.com/webpack/webpack-dev-server/releases)
- [Changelog](https://github.com/webpack/webpack-dev-server/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack/webpack-dev-server/commits/v3.1.11)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]