New style general blogposts

What will happen

As discussed with @marcia @timzallmann we will implement a first revision into improving the overall blogpost style used on about.gitlab.com/blog.

The way we will do this is by doing a second UX and Frontend revision on the stylistic css changes introduced in https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/6041.

These changes will merge into the branch of https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/6041. After that it will be merged into master.

What needs to be done

• Stylistic changes of https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/6041 need to be made the default styling, instead of using extra_css.
• https://gitlab.com/gitlab-com/www-gitlab-com/issues/1480 will require a decent set of H1 to H6 restyling, which looks great with body text.
• Needs to be approved by product etc, after which we can merge.

Target launch: 23 June

Closes https://gitlab.com/gitlab-com/www-gitlab-com/issues/1480 Closes https://gitlab.com/gitlab-com/www-gitlab-com/issues/1518