Add 'eslint-plugin-header' to check license

Add eslint rule to verify that source files have the required license
header.
Convert `.eslintrc.json` to `.eslintrc.cjs` to add header plugin
configuration.

Gitlab: #45
Change-Id: I7a2331d68a418be35e6bea425eede92087451980
diff --git a/client/.eslintrc.json b/client/.eslintrc.json
index 4188353..e94691b 100644
--- a/client/.eslintrc.json
+++ b/client/.eslintrc.json
@@ -4,7 +4,11 @@
       "version": "detect"
     }
   },
-  "extends": ["plugin:react/recommended", "plugin:react-hooks/recommended", "../.eslintrc.json"],
+  "extends": [
+    "plugin:react/recommended",
+    "plugin:react-hooks/recommended",
+    "../.eslintrc.cjs"
+  ],
   "rules": {
     "react-hooks/exhaustive-deps": "error",
     "react/display-name": "off",