[lint] Update context for linting frontend files:

- Lint files when pushing on the _frontend_ branch.
- Lint files when eslint or stylelint config files are updated.
- Use _strict_ parsing of JS files by default. This should make the JS linting test to fail.

.eslintrc.js

@@ -6,5 +6,9 @@ module.exports = {
         "es6": true
     },
 
-    "extends": "eslint:recommended"
+    "extends": "eslint:recommended",
+
+    "rules": {
+        "strict": "error"
+    }
 };