Revert "Remove the `:coffee` and `:coffeescript` Haml filters"

This reverts commit ae7de2f8.

Several files in EE still use the :coffeescript filter and I don't have time to fix them before the CE-to-EE merge.

See https://gitlab.com/gitlab-org/gitlab-ee/builds/553647

I will reintroduce this change in a future MR.