Restore bootstrap panel classes for blog posts

All the bootstrap panels are broken in blog posts.

Example

(from https://about.gitlab.com/2016/09/14/gitlab-live-event-recap/)

How they should look like

(from https://about.gitlab.com/handbook/marketing/developer-relations/technical-writing/markdown-guide/#colorful-sections):

Phil, I don't have a quick solution for this, but I'm sure it's easy for you to find out 😸 Thank you!!

Please let me know if you need any help.