Disallow self assignment of properties (no-self-assign)

This rule was actually enabled in a prior upgrade of ESLint. Let's set the option explicitly nonetheless.

Fixes: #1186

eslintrc.json

@@ -1,6 +1,7 @@
 {
   "extends": ["standard", "standard-jsx"],
   "rules": {
-    "quotes": ["error", "single", { "avoidEscape": true }]
+    "quotes": ["error", "single", { "avoidEscape": true }],
+    "no-self-assign": ["error", { "props": true }]
   }
 }