CI - Manual pipelines for MRs

• Developers pipelines are now manual
• The first job/stage acts as a gateway to continue all the other jobs/stages

Context

Reduce the CI load (and overall coast) of our infrastructure at AWS

Side effects

• If you rebased just before assigning to Marge-bot, Marge-bot will not trigger a new pipeline and end-up with a blocked pipeline (could be fixed in Marge-bot's python code)
• A lot of pipelines in blocked status in the namespace (harder to filter/browse), performance/API wise GitLab support confirmed it's not an issue

Note

This MR is actually reopened from the revert of !5518 (merged), we noted, after merging a regression is the followinf scenario (quoted from @raphael-proust analysis )

In the case:

1. A MR was assigned to margebot
2. margebot says "conflicts, you rebase"
3. margebot assigns original author
4. author pushes a rebased version
5. pipeline with manual trigger is created
6. author assigns margebot, stuck

Proposal : I think that margebot might need to push the manual trigger button sometimes