refactor: split domain code from servlet code

[evantill]

First step on PlantUmlServlet.

The main goal is to spliut the code depnding on the Servlet API from the rest of the code. This way, it will be possible to use an other adpter layer like a rest api for example.

Note: I will add more commits.

[evantill]

Will the checks run the tests ?

[evantill]

running the same mvn task does not raise error on my computer

mvn --batch-mode -f pom.jdk8.xml -D java.net.useSystemProxies=true -Dapache-jsp.scope=compile clean package

Can someone help me please ?

[evantill]

will be refactored on thymeleaf branch