.gitlab-ci.yml: Use a custom conf.js

Allows to have a menu with access to existing fused files.
7 jobs for feature-c/improve_file_menu in 6 minutes and 43 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test Tools
passed #117848
test_tools

00:00:05

 
  Test Quality
passed #117849
web
code_quality

00:00:13

 
  Test Unit
passed #117850
web
test_browser_unit

00:00:10

 
  Deploy Review
passed #117851
web
deploy_review

00:00:16

manual #117852
web allowed to fail manual
stop_deploy_review
 
  Test Functional
passed #117853
web
test_browser-functional

00:03:32

 
  Test Functional External
passed #117854
web
test_browser-functional-external

00:02:25