Code style (#696)

* Remove Guava dependency from WebGoat

* Add Checkstyle to the project with very basic standards so we have a
style across lessons. It does not interfere with basic Intellij formatting

webgoat-container/pom.xml

@@ -116,11 +116,6 @@
             <artifactId>commons-lang3</artifactId>
             <version>${commons-lang3.version}</version>
         </dependency>
-        <dependency>
-            <groupId>com.google.guava</groupId>
-            <artifactId>guava</artifactId>
-            <version>${guava.version}</version>
-        </dependency>
         <dependency>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-security</artifactId>