Add CI config for GitLab

.gitlab-ci.yml

+variables:
+  MIRROR_REPO: git@github.com:thm-projects/arsnova-backend.git
+  WAR_FILE: target/arsnova-backend-*.war
+
+stages:
+  - analysis
+  - test
+  - build
+  - deploy
+  - synchronization
+
+cache:
+  paths:
+    - .m2/
+
+sync_mirror:
+  stage: synchronization
+  when: always
+  except:
+    - production
+    - staging
+  allow_failure: true
+  script:
+    - git clone --bare $CI_BUILD_REPO mirror.git
+    - cd mirror.git
+    - git update-ref -d refs/tags/production
+    - git update-ref -d refs/tags/staging
+    - git push --mirror $MIRROR_REPO
+
+sonar:
+  stage: analysis
+  only:
+    - master
+  tags:
+    - maven
+  allow_failure: true
+  script:
+    - mvn sonar:sonar
+
+checkstyle:
+  stage: analysis
+  tags:
+    - maven
+  script:
+    - mvn checkstyle:checkstyle
+
+test:
+  stage: test
+  tags:
+    - maven
+  script:
+    - mvn test
+
+package:
+  stage: build
+  tags:
+    - maven
+  artifacts:
+    paths:
+      - $WAR_FILE
+  script:
+    - mvn package -DskipTests
+
+tomcat_production:
+  stage: deploy
+  only:
+    - production
+  script:
+    - curl --upload-file $WAR_FILE https://$PROD_TOMCAT_USER:$PROD_TOMCAT_PASSWORD@$PROD_TOMCAT_HOST/manager/deploy?path=%2Fapi
+
+tomcat_development:
+  stage: deploy
+  only:
+    - staging
+  script:
+    - curl --upload-file $WAR_FILE https://$DEV_TOMCAT_USER:$DEV_TOMCAT_PASSWORD@$DEV_TOMCAT_HOST/manager/deploy?path=%2Fapi