Improve validation of messages.yml

[Marcono1234]

As shown by #121, the validation of the messages.yml file is currently not checking whether projects referenced in quick_links and messages have an entry in project_id.

Note however that mctest is at the moment referenced by messages despite having no quick_links and project_id entry.

[SPGoding]

The validation of messages.yml is implemented by 1) converting the YAML file to a JSON file and 2) using JSON schemas to validate the generated JSON file. So I'm not sure if it's possible to do a complicated validation like this 🤔