Add new Sagan client application

This commit brings the new JavaScript files and static resources for the
client application.

Everything is bundled and optimized with a webpack build. The Gradle
build is triggering the NPM toolchain and packaging the sagan-client
module as a JAR.

sagan-client/.gitignore

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+node_modules
+npm-debug.log
+package-lock.json

sagan-client/README.md

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+# Sagan client module
+
+This module holds all web resources that make the client application of spring.io:
+
+* JavaScript modules
+* CSS styles
+* images and fonts
+* front-end dependencies
+
+This module is using several tools for its own build system:
+
+* [node.js and npm](http://nodejs.org)
+* [Webpack](https://webpack.js.org/)
+
+## Build requirements
+
+Node.js is brought by the Gradle build itself, so you don't need it to build the project.
+But if you want to work on the `sagan-client` module, installing a recent version of [node.js](http://nodejs.org)
+is a good idea. Usage of the [Node Version Manager (nvm)](https://github.com/nvm-sh/nvm) is perfectly fine.
+
+## Making changes in sagan-client
+
+When running the application with the `SiteApplication` class in your IDE, resources in sagan-client are served
+directly from resources built in the sagan-client module, so you can keep the Java application running
+and re-build the client by running  `npm run build` on the command line.
+
+For this to work, your IDE should be configured to consider the `sagan-site` module as its working directory. Check out
+the [run the site locally](https://github.com/spring-io/sagan/wiki/Run-the-site-locally) section on the wiki.
+
+## Details about the JavaScript build
+
+If you want to know more about the JavaScript build, this chapter will help you; reading this is not required.
+
+The JavaScript application can be built manually with (the build result is located in the `build/dist` folder):
+
+```
+$ npm run build
+```
+
+This will trigger the webpack build, which bundles and optimizes the JavaScript, CSS and images.
+Check the `webpack.config.js` file for more details.
+
+### Node.js and npm
+
+npm is the node package manager; it installs required dependencies in the `node_modules` directory.
+Check the `package.json` file to find:
+
+- all dependencies and their versions in `devDependencies`
+- all available `scripts` that you can run with `npm run scriptname`
+
+Note: we make extensive use of npm scripts so you don't have to install binaries globally on your system's PATH.
+npm dynamically adds binaries listed in `node_modules/.bin` to its own PATH.

sagan-client/build.gradle

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+plugins {
+	id 'com.moowork.node' version '1.3.1'
+}
+
+apply plugin: 'java'
+apply plugin: 'com.moowork.node'
+
+node {
+    version = '13.2.0'
+    npmVersion = '6.13.1'
+    download = true
+}
+
+def jsBuildDir = project.buildDir.absolutePath + '/dist'
+
+jar {
+	from jsBuildDir
+	eachFile { details ->
+		details.path = details.path.startsWith('META-INF') ?: 'static/'+details.path
+	}
+	includeEmptyDirs = false
+}
+
+task npmBuild(dependsOn: npmInstall, type: NpmTask) {
+	inputs.file 'package.json'
+	inputs.dir('src')
+	outputs.dir jsBuildDir
+	args = ['run', 'build']
+}
+
+jar.dependsOn npmBuild

sagan-client/package.json

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+{
+  "name": "sagan",
+  "description": "Client application for spring.io",
+  "private": true,
+  "repository": {
+    "type": "git",
+    "url": "http://github.com/spring-io/sagan.git"
+  },
+  "dependencies": {
+    "@fancyapps/fancybox": "^3.5.7",
+    "@fortawesome/fontawesome-free": "^5.12.0",
+    "bulma": "^0.8.0",
+    "jquery": "^3.4.1"
+  },
+  "devDependencies": {
+    "clean-webpack-plugin": "^3.0.0",
+    "copy-webpack-plugin": "^5.0.5",
+    "css-loader": "^3.2.1",
+    "file-loader": "^5.0.2",
+    "mini-css-extract-plugin": "^0.8.0",
+    "optimize-css-assets-webpack-plugin": "^5.0.3",
+    "style-loader": "^1.0.1",
+    "terser-webpack-plugin": "^2.2.1",
+    "webpack": "^4.41.2",
+    "webpack-cli": "^3.3.10"
+  },
+  "scripts": {
+    "build": "webpack"
+  }
+}

sagan-client/src/app/admin.js

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+import 'bulma/css/bulma.css'
+import '../css/admin.css'

sagan-client/src/app/blog.js

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+import '../css/blog.css'

sagan-client/src/app/guide.js

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+import '../css/guide.css'