Add `Open` and `Closed` category tabs

ee/app/views/shared/issuable/_epic_nav.html.haml

@@ -3,4 +3,12 @@
 - display_count = local_assigns.fetch(:display_count, :true)
 
 %ul.nav-links.epics-state-filters
+  %li{ class: active_when(params[:state] == 'opened') }>
+    = link_to page_filter_path(state: 'opened', label: true), id: 'state-opened', title: "Filter by #{page_context_word} that are currently opened.", data: { state: 'opened' } do
+      #{issuables_state_counter_text(type, :opened, display_count)}
+
+  %li{ class: active_when(params[:state] == 'closed') }>
+    = link_to page_filter_path(state: 'closed', label: true), id: 'state-closed', title: 'Filter by issues that are currently closed.', data: { state: 'closed' } do
+      #{issuables_state_counter_text(type, :closed, display_count)}
+
   = render 'shared/issuable/nav_links/all', page_context_word: page_context_word, counter: issuables_state_counter_text(type, :all, display_count)