Drop GFM support in Issue/MergeRequest in Milestone

Description

Drop GFM support in Issue/MergeRequest in Milestone page to speed up page rendering.

Proposal

Milestone page with many issues and/or merge requests (typically 30-100) is extremely slow (meta: #33885 (moved)) due to GFM support of their title there. Even on Issues and Merge Requests page, the title of each issue/merge request does not have GFM support, so we should drop GFM support on Milestone page both for the consistency and performance.

Research

• https://gitlab.com/gitlab-org/gitlab-ce/issues/33885#note_33427661
• https://gitlab.com/gitlab-org/gitlab-ce/issues/33885#note_33427891

Links / references

Meta: #33885 (moved)

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/label ~"feature proposal" milestones ~performance backend