Backport Extract epic nav into separate partial for easier CE => EE

app/views/shared/issuable/_nav.html.haml

 - type = local_assigns.fetch(:type, :issues)
 - page_context_word = type.to_s.humanize(capitalize: false)
-- issuables = @issues || @merge_requests
 
 %ul.nav-links.issues-state-filters
   %li{ class: active_when(params[:state] == 'opened') }>
@@ -20,6 +19,4 @@
       = 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)}
 
-  %li{ class: active_when(params[:state] == 'all') }>
-    = link_to page_filter_path(state: 'all', label: true), id: 'state-all', title: "Show all #{page_context_word}.", data: { state: 'all' } do
-      #{issuables_state_counter_text(type, :all)}
+  = render 'shared/issuable/nav_links/all', page_context_word: page_context_word, counter: issuables_state_counter_text(type, :all)

app/views/shared/issuable/nav_links/_all.html.haml

+- page_context_word = local_assigns.fetch(:page_context_word)
+- counter = local_assigns.fetch(:counter)
+
+%li{ class: active_when(params[:state] == 'all') }>
+  = link_to page_filter_path(state: 'all', label: true), id: 'state-all', title: "Show all #{page_context_word}.", data: { state: 'all' } do
+    #{counter}