Make instance names in into links to user list in the instance (#4924)

app/views/admin/instances/_instance.html.haml

@@ -1,6 +1,6 @@
 %tr
   %td.domain
-    = instance.domain
+    = link_to instance.domain, admin_accounts_path(by_domain: instance.domain)
   %td.count
     = instance.accounts_count
   %td