Use merge request prefix symbol in event feed title

What does this MR do?

Use merge request prefix symbol in event feed title. (see https://gitlab.com/gitlab-org/gitlab-ce/issues/36267#note_109566469)

What are the relevant issue numbers?

Closes #36267 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides