Rework CI

• only check syntax for PRs
• add a dev branch to update tools on dev instance
• master updates both preprod and prod
• preprod branch can be deleted

and also :

• remove yamllint as it's redundant with pykwalify + it doesn't like the yaml produced by the update script (a matter of taste https://github.com/yaml/pyyaml/issues/234)
• little changes to auto update MRs

TODO:

• make master the default branch
• allow new users to run ci pipeline on their MR
• create dev branch
• update README

Fixes #12 (closed), #9 (closed)