Add eslint rules for React hooks

These make components with simple state (only a few variables) easier to
read and maintain. See https://reactjs.org/docs/hooks-intro.html for
details. Hook are available since React 16.8, and we already depend on 16.10.

We can't use hooks in our actual code, as our only `Application`
component needs a constructor. But this enables the ESLint plugin to
guide developers to the right path if they use hooks.

Closes #239

.eslintrc.json

@@ -13,7 +13,7 @@
         },
         "sourceType": "module"
     },
-    "plugins": ["flowtype", "react"],
+    "plugins": ["flowtype", "react", "react-hooks"],
     "rules": {
         "indent": ["error", 4,
             {
@@ -28,6 +28,9 @@
         "react/jsx-indent": ["error", 4],
         "semi": ["error", "always", { "omitLastInOneLineBlock": true }],
 
+        "react-hooks/rules-of-hooks": "error",
+        "react-hooks/exhaustive-deps": "error",
+
         "camelcase": "off",
         "comma-dangle": "off",
         "curly": "off",