Fix broken merge request views

What does this MR do?

Teach render_ce to also search for full templates. It seems like https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6032 introduced an error which leads to 500s when displaying merge requests:

Does this MR meet the acceptance criteria?

• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

• Closes gitlab-qa#270 (closed)
• Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/6363