Clicking Merge requests assigned to me option in the search bar leading to 404 error when merge requests are disabled

It was suggested in !128376 (comment 1500653172) to not show merge request related links in the search bar/results if the project has disabled merge requests. Currently, they just return 404s.

Possible Solution

Do not render the Merge requests assigned to me, Merge requests I'm reviewing and Merge requests I've created links in projects which have merge requests disabled.