Add and apply Style/RedundantStringEscape rule

This rule was added in rubocop 1.37.0. It's particularly useful in the
background image spec, since now there's one less backslash to decipher
when reading the code :).

.rubocop.yml

@@ -691,6 +691,9 @@ Style/RedundantReturn:
 Style/RedundantSelf:
   Enabled: true
 
+Style/RedundantStringEscape:
+  Enabled: true
+
 Style/SafeNavigation:
   Enabled: true