Fix misc. linter issues (comments, variable naming, etc.)

We've accumulated a few linter issues around comments and a couple of
variable names.

While none of them is major, this patch cleans them up so it's easier to
go through the linter output, and we can start being more strict about
it.

internal/courier/mda_test.go

@@ -113,6 +113,8 @@ func TestSanitize(t *testing.T) {
 		// http://www.user.uni-hannover.de/nhtcapri/bidirectional-text.html
 		// We allow them, they're the same on both sides.
 		{"١٩٩٩–١٢–٣١", "١٩٩٩–١٢–٣١"},
+
+		//lint:ignore ST1018 The use of a literal U+200C is intentional.
 		{"موزه‌ها", "موزه\u200cها"},
 	}
 	for _, c := range cases {