fix(ci): pipeline only on merge request

Now runs the pipeline only once a MR is open.

Fixes #83
Status Job ID Name Coverage
  Check
passed #475954
docker
check_back

00:00:22

passed #475955
docker
check_front

00:01:11

 
  Test
passed #475956
docker
test_back

00:00:42

88.0%
passed #475957
docker
test_frontend

00:00:59

 
  Lint
passed #475959
docker
eslint

00:01:05

passed #475958
docker
flake8

00:00:12

 
  Svg Gen Docu
passed #475960
docker
generate_UML_svg

00:00:13

 
  Documentation
passed #475961
docker
pages

00:00:12

 
  Deploy
passed pages:deploy

00:00:00