Enable gitlab-pages daemon to send precompressed br files

Release notes

If you are looking for ways to improve your GitLab Pages load times, we are excited to announce a great community contribution from feistel and [Ambyjkl](https://gitlab.com/Ambyjkl], where we add support for brotli compressed files. This will help lower the required bandwidth per page, improving your site load times. https://docs.gitlab.com/ee/user/project/pages/introduction.html#serving-compressed-assets

Problem to solve

Brotli compressed files are not supported on GitLab Pages.

Proposal

The brotli compression is now supported by modern browsers. It can already be used in gulp, webpack, as well as Jekyll through jekyll-brotli. If enabled, it could help lower the required bandwidth per page.