Add "Go back" link in admin verifications

It isn't very intuitive that this link points to the stats page, but
since it's the only page linking to it and there's no link pointing to
it in the admin navigation, I guess it's better than offering no clue at
all of the current whereabouts.

app/views/admin/verifications/index.html.erb

@@ -1,3 +1,5 @@
+<%= back_link_to admin_stats_path %>
+
 <h2><%= t("admin.verifications.index.title") %></h2>
 
 <%= render "admin/shared/user_search", url: search_admin_verifications_path %>

spec/system/admin/verifications_spec.rb

@@ -9,6 +9,7 @@ describe "Incomplete verifications", :admin do
 
     visit admin_verifications_path
 
+    expect(page).to have_link "Go back", href: admin_stats_path
     expect(page).to have_content(incompletely_verified_user1.username)
     expect(page).to have_content(incompletely_verified_user2.username)
     expect(page).not_to have_content(never_tried_to_verify_user.username)