enable Slippers blog template

Why is this change being made?

This MR applies a new blog template designed by the Digital Experience team to all blog posts in the marketing blog.

This template was built in Slippers first here, and then we are applying the template to the marketing blog with this MR. This MR will update all marketing posts, currently 1298 webpages, with the new template.

QA test on top 10 blog post (based on unique visits) before deployment:

• /blog/2021/03/10/new-git-default-branch-name/index.html
• /blog/2021/02/22/gitlab-kubernetes-agent-on-gitlab-com/index.html
• /blog/2021/02/05/ci-deployment-and-environments/index.html
• /blog/2016/07/29/the-basics-of-gitlab-ci/index.html
• /blog/2020/12/15/deploy-aws/index.html
• /blog/2021/02/22/pipeline-editor-overview/index.html
• /blog/2020/12/10/basics-of-gitlab-ci-updated/index.html
• /blog/2018/06/07/keeping-git-commit-history-clean/index.html
• /blog/2016/12/14/continuous-delivery-of-a-spring-boot-application-with-gitlab-ci-and-kubernetes/index.html
• /blog/2018/03/05/gitlab-for-agile-software-development/index.html

Closes https://gitlab.com/gitlab-com/marketing/inbound-marketing/growth/-/issues/1119

Closes https://gitlab.com/gitlab-com/marketing/inbound-marketing/growth/-/issues/1146

Related https://gitlab.com/gitlab-com/marketing/inbound-marketing/growth/-/issues/765

TODO

• Clean up development slippers blog instance in sites/marketing/config.rb
• Clean up test files in sites/marketing/source/slippers/posts/
• Demonstrate the new blog template on an existing blog post (or a whole set of posts if we want to test it more thoroughly).
• Shared requirements have been met.
• Figure out an initial QA strategy
• Add documentation about the new blog template to the handbook.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI