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 !6041 (merged).

These changes will merge into the branch of !6041 (merged). After that it will be merged into master.

What needs to be done

• Stylistic changes of !6041 (merged) need to be made the default styling, instead of using extra_css.
• #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

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Closes #1480 Closes #1518 (closed)