Change capitalization of Merge Requests in the UI

According to the latest Pajamas style standards (assuming this MR gitlab-org/gitlab-services/design.gitlab.com!1984 (merged) is merged and per this comment thread gitlab-org/gitlab-services/design.gitlab.com!1984 (comment 376987680)):

The terms "merge request"/"merge requests" should use lowercase, except in the context of starting a UI label or sentence, where the first letter would be capitalized, as "Merge request" or "Merge requests".

Proposal

Change "Merge Requests" to "Merge requests" in the GitLab navigation menu and breadcrumb of the MR list page (at /merge_requests).