Add checkstyle checking during Maven build

So far no rules are activated and it just puts the infrastructure in places.
Rules can be added in separately after discussion among the developers, along
with fixing all the violations of the newly activated rules.

checkstyle.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC
+        "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+        "https://checkstyle.org/dtds/configuration_1_3.dtd">
+
+<module name="Checker">
+</module>

pom.xml

@@ -211,6 +211,11 @@
                     <artifactId>maven-war-plugin</artifactId>
                     <version>3.4.0</version>
                 </plugin>
+                <plugin>
+                    <groupId>org.apache.maven.plugins</groupId>
+                    <artifactId>maven-checkstyle-plugin</artifactId>
+                    <version>3.6.0</version>
+                </plugin>
             </plugins>
         </pluginManagement>
         <plugins>
@@ -312,6 +317,23 @@
                     </execution>
                 </executions>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-checkstyle-plugin</artifactId>
+                <configuration>
+                    <configLocation>checkstyle.xml</configLocation>
+                    <failOnViolation>true</failOnViolation>
+                </configuration>
+                <executions>
+                    <execution>
+                        <id>validate</id>
+                        <phase>process-sources</phase>
+                        <goals>
+                            <goal>check</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
         </plugins>
     </build>
 </project>