test: add and configure Jest for running tests of React components

The following dependencies have been installed:
- jest: the testing framework
- babel-jest: integration for Babel (https://jestjs.io/docs/en/getting-started#using-babel),
  otherwise tests fail with "Support for the experimental syntax 'jsx' isn't currently enabled"
- @testing-library/react: the library for testing React components
- @testing-library/jest-dom: useful matches for inspecting DOM
- react and react-dom: in order to import React in tests

All the dependencies were installed by the command:
./mvnw -Pfrontend frontend:npm -Dfrontend.npm.arguments='install --save-dev jest babel-jest @testing-library/react @testing-library/jest-dom react@16.8.6 react-dom@16.8.6'

In order to execute (or execute and watch) the tests, run:
    ./mvnw -Pfrontend frontend:npm -Dfrontend.npm.arguments='test'
or
    ./mvnw -Pfrontend frontend:npm -Dfrontend.npm.arguments='run watch-test'

Alternatively, it's possible to use npx:
   cd src/main/frontend
   npx jest
or
   npx jest --watch

Note that at the moment of this commit, there is no tests yet, so the previous command will fail.

While I marked React as an external (see webpack.config.js), all bundles got additional 5 Kb and I
couldn't figure out why. Perhaps, it's how it should work, but I'm not sure as I expected that
bundles size will stay the same.

Useful links:
- https://jestjs.io
- https://github.com/testing-library/react-testing-library
- https://github.com/testing-library/jest-dom
- https://www.newline.co/@dmitryrogozhny/how-to-start-using-react-testing-library--0e7695e8

Part of #1484

Author: php-coder

pom.xml

@@ -645,7 +645,12 @@
 		<!-- Redefine default value from spring-boot-dependencies -->
 		<postgresql.version>9.4.1212.jre7</postgresql.version>
 
-		<!-- Don't forget to update version in the ResourceUrl class and in the src/main/webapp/WEB-INF/views/series/info.html -->
+		<!--
+			Don't forget to update version in the following places:
+			- ResourceUrl class
+			- src/main/webapp/WEB-INF/views/series/info.html
+			- src/main/frontend/package.json
+		-->
 		<react.version>16.8.6</react.version>
 
 		<resources.plugin.version>3.2.0</resources.plugin.version>

src/main/frontend/babel.config.js

@@ -1,3 +1,4 @@
+// Note that this configuration is used by Webpack (babel-loader) and Jest
 module.exports = {
 	presets: [ "@babel/preset-react", "@babel/preset-env" ]
 };

src/main/frontend/jest.config.js

@@ -0,0 +1,5 @@
+// @todo #1484 Document Jest usage
+module.exports = {
+	// https://jestjs.io/docs/en/configuration#watchman-boolean
+	"watchman": false
+}