Use white-space: pre-line for invoice notes and payment instructions

I want the `white-space: pre` to preserve the newline characters that
users may have used, but this prevents line wrapping and long lines are
not confined within the page margins.

`pre-line` preserves the newlines, but collapses spaces and tabs, and
wraps long text, which is more what i want.

web/static/invoice.css

@@ -56,7 +56,7 @@
 }
 
 .invoice .notes, .invoice .payment-instructions {
-    white-space: pre;
+    white-space: pre-line;
     text-align: right;
 }