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.

.rubocop.yml

@@ -66,6 +66,9 @@ Layout/IndentationWidth:
 Layout/LeadingEmptyLines:
   Enabled: true
 
+Layout/LineEndStringConcatenationIndentation:
+  Enabled: true
+
 Layout/LineLength:
   Max: 110
   Severity: refactor