Add a new stage 'check input' to our CI configuration to check whether
our input files contain errors.
The first check is for the YAML files whether they contain errors or
warnings according to 'yamllint'.
Add 'yamllint' config file to not consider too long lines as errors.
And fix the errors/warnings in 'salsa-ci.yml' itself.
Apparently job names can contain space, so rename 'build-yamls' to
'build yamls'.
Diederik de Haas