Fix issue description syntax highlighting and math rendering

What does this MR do?

Fix issue description syntax highlighting and math rendering

Before	After

Are there points in the code the reviewer needs to double check?

Regressed in https://gitlab.com/gitlab-org/gitlab-ce/commit/5a95d6f8dae00b31b694759c6ddbf6d83b1a7890#3a15290e7696397415523e0b664aceb3dd4010ae_0_46 (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11242) where the ref no longer existed and we called $(undefined).renderGFM()

Why was this MR needed?

Syntax highlighting and math not rendering in issue description on page load or from real-time update.

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #33877 (closed)