lint: Enable comma-dangle eslint rule.

This styling doesn't work on IE8 and older browsers, but we've
basically abandoned IE8 as the newest browser we don't support anyway.

If we wanted to restore IE8 support, it wouldn't be hard to reverse
this transformation as part of our static asset build process.

.eslintrc.json

@@ -140,7 +140,13 @@
         "no-caller": 2,
         "no-bitwise": 2,
         "no-catch-shadow": 2,
-        "comma-dangle": 0,
+        "comma-dangle": ["error", {
+            "arrays": "always-multiline",
+            "objects": "always-multiline",
+            "imports": "always-multiline",
+            "exports": "always-multiline",
+            "functions": "never"
+        }],
         "no-console": 1,
         "no-control-regex": 2,
         "no-debugger": 2,