Move babel config into separate file

This works better with the current eslint loader, as then eslint and
babel can look at the same configuration. See
<https://babeljs.io/docs/en/config-files>.

This is similar to .eslintrc.json.

.babelrc.json

@@ -0,0 +1,14 @@
+{
+    "presets": [
+        ["@babel/env", {
+            "targets": {
+                "chrome": "57",
+                "firefox": "52",
+                "safari": "10.3",
+                "edge": "16",
+                "opera": "44"
+            }
+        }],
+        "@babel/preset-react"
+    ]
+}