Navigation bar issuables counters reflects dashboard issuables counters

Adds a 2 minute cache on navigation bar issuables counters.

The counters on dashboard/issues and dashboard/merge_requests are cached while the navigation bar issuables counters were not. Users read incoherent issuables count throughout the application.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/23882

See merge request !7368