Add Gitlab CI to siteimprove module

mohd sahzad → made their first commit to this issue’s fork.

I tried to keep the changes as minimal as possible, Eslint, Stylelint and CSPell issues should be handled on separate issue.

Have made a few comments on the MR, of particular concern is the removal of some of the logical structures.

The most common procedure I have seen on other modules has been to just have this issue turn on CI then create follow-ups for all of the issues arising from the tests. I suggest we do that here too.