Add haml-linter for the marketing site
Why?
The marketing site is getting bigger and maintaining it will require due diligence from everyone involved, but this should give us a good step in the right direction, this way MR's will be of higher quality and consistent through the repo.
I took the liberty to work on a branch and activate the linter there with the default values, what I got was not pretty but we could choose to deactivate some of the linters if we decide to move forward with the implementation.
Some of the files are here in this snippet (file is too large for the description or a comment) https://gitlab.com/gitlab-com/www-gitlab-com/snippets/1677693