Standardize a way to retrieve label for issue list pages
These are follow up changes to an inconsistent code changes in !2854 (merged).
The approach to retrieve label for Projects::IssuesController#index and Projects::MergeRequestsController#index isn't consistent with the approach for GroupsController#issues/DashboardController#issues and GroupsController#merge_requests/DashboardController#merge_requests. This MR makes them consistent.