Navigation: Rename "Board" to "Boards" in Issues sub navigation

Description

Category "Cosmetics", it doesn't get much smaller than this. With the newly added multiple issue boards in GitLab EE ❤, the sub-nav item "Board" should be renamed to "Boards". I checked with all other navigation items, this is the only inconsistent name to me right now.

Proposal

Rename "Board" to "Boards" in "Issues" sub navigation (for GitLab EE).