Fixing Jest for Babel 7 in Plugins.

Due to a change in Babel 7 that affects resolution of babel configs[1]
(`.babelrc` is now a hierarchical config, while `babel.config.js` is
not), source files outside of the source root of jest will not be
transpiled. That means, plugins running tests were not able to
require/import source files of core.

With this change, `.babelrc` will be renamed to `babel.config.js`, so it
is still used when a plugin imports a file from core.

[1]: jestjs/jest#6053 (comment)

graylog2-web-interface/babel.config.js

@@ -0,0 +1,13 @@
+module.exports = {
+  presets: ['@babel/env', '@babel/react', '@babel/flow'],
+  plugins: [
+    '@babel/plugin-syntax-dynamic-import',
+    '@babel/plugin-proposal-class-properties',
+    'add-module-exports',
+  ],
+  env: {
+    test: {
+      plugins: ['babel-plugin-dynamic-import-node'],
+    },
+  },
+};