Reconcile project templates with Auto DevOps

Description

Project templates are shipped with their own .gitlab-ci.yml configuration. This is very specific to the language, but it doesn't implement all the functionalities that are available in Auto DevOps.

We should ensure Auto DevOps is working well with project templates.

Proposal

Improve project templates to make them working fine with Auto DevOps. Once it is done, we can remove the explicit .gitlab-ci.yml file or just replace it with Auto DevOps template.

Things to do:

1. ensure apps are showing an "Hello world" page for / requests, otherwise readiness probes will fail
2. remove .gitlab-ci.yml
3. Add notes in the README.md to explain that you need to enable Auto DevOps or to load the Auto DevOps template if you want to run pipelines
4. Ensure there is one test and that it runs correctly

Templates

• improve Spring template https://gitlab.com/gitlab-org/project-templates/spring/
• improve Express template https://gitlab.com/gitlab-org/project-templates/express/
• improve Rails template https://gitlab.com/gitlab-org/project-templates/rails/

(OPTIONAL) modify the vendoring process

• make repository flat
• make files owned by a service account

Vendoring

• vendor templates