Add SyntaxHighlightFilter to markup pipeline

What does this MR do?

As described in https://gitlab.com/gitlab-org/gitlab-ce/issues/20023#note_86125862 syntax highlighting for code blocks doesn't work on *.rst files since the code class is missing on the <pre> tag.

The first approach which simply added the code class failed because we are removing the class with our HTML Pipeline eventually as described in gitlab-markup!12 (comment 147191914)

This MR enables syntax highlighting on the markup pipeline as proposed by @brodock in gitlab-markup!12 (comment 147193111)

Result

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/20023

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer