Fix the "Edit in Web IDE" link for blog posts

Problem to solve

Due to the blog part of the website already being moved into a sub-project in the www-gitlab-com repo as part of the monorepo refactoring that is in progress there is a symlink for the old source/blog/posts directory to the new sites/blog/source/blog/blog-posts directory.

The Edit in Web IDE link is not aware of this for the blog and we need to fix this as it leads to problems.

More context: https://gitlab.com/gitlab-com/www-gitlab-com/-/issues/7394#note_333822623

Proposed solution

The link generated is in source/includes/layout/footer.html.haml This partial is included in all layout files. Looks like blog uses the source/layouts/default.haml layout file.

We can duplicate that layout file to something called blog.haml and included a custom footer with the correct generated link.