Add empty state for epics

ee/app/views/groups/epics/_epics.html.haml

-- page_title "Epics"
-
-%ul.content-list.issues-list.issuable-list

ee/app/views/groups/epics/index.html.haml

 - page_title "Epics"
 
+- if @epics.to_a.any?
+  -# TODO: Add epics list
+- else
+  = render 'shared/empty_states/epics'

ee/app/views/shared/empty_states/_epics.html.haml

+.row.empty-state
+  .col-xs-12
+    .svg-content
+      = sprite_icon('illustrations/epics.svg')
+  .col-xs-12.text-center
+    .text-content
+      %h4
+        Epics let you manage your portfolio of projects more efficiently and with less effort.
+      %button.btn.btn-new{ type: 'button' }
+        New epic