gitlab-foss

Replace Haml with Hamlit

What this change does:
- It replaces HAML with [Hamlit](https://github.com/k0kubun/hamlit), a much more efficient implementation of HAML.
- It removes `haml-rails`.
- It adds `hamlit.rb` and removes `haml.rb`.
- It makes things faster and less memory-hungry!

See also #13201.

See merge request !3666