`Board` should be `Boards` in new navigation

Same thing as https://gitlab.com/gitlab-org/gitlab-ee/issues/3015, but now for the latest master branch on EE.

added regression label

added Discussion label

changed milestone to %10.0

added frontend label

assigned to @iamphill

mentioned in merge request !2890 (merged)

mentioned in commit 8b4bec59

closed via commit 36f02298

mentioned in commit 36f02298

mentioned in commit 6239c814