Cleanup YAML config for consistentency and automation

If we want to start doing some automation, we should be sure our YAML files are coherent and follow some guidelines.

After cleaning them, we could even add a CI to make sure the styling is enforced.

I wrote a quick and dirty script that uses ruamel.yaml to see which files it cannot write back as the original.

The errors it produces are more or less expected:

Failed to process projects/common [Errno 2] No such file or directory: 'projects/common/config'
Failed to process projects/firefox-l10n expected a single document in the stream
  in "projects/firefox-l10n/config", line 2, column 1
but found another document
  in "projects/firefox-l10n/config", line 16, column 1

And the diff is also clean enough.

I'm not sure of

yaml.indent(mapping=2, sequence=4, offset=2)
yaml.width = 1024

(BTW, the width isn't enough for projects/config/rust 😨).

/cc @boklm