Add Layout/LineEndStringConcatenationIndentation rule

This rule was added in Rubocop 1.18.0, but we didn't add it back then.
Since we're applying it most of the time, we might as well be consistent
and apply it everywhere.

spec/system/admin/administrators_spec.rb

@@ -36,7 +36,7 @@ describe "Admin administrators" do
     visit admin_administrators_path
 
     confirmation = "Are you sure? This action will delete "\
-      "\"#{user_administrator.name}\" and can't be undone."
+                   "\"#{user_administrator.name}\" and can't be undone."
 
     within "#administrator_#{user_administrator.id}" do
       accept_confirm(confirmation) { click_button "Delete" }