Configure YAML linting (OSOE-759)

[Piedone]

After Lombiq/NodeJs-Extensions#11, configure the YAML linting rules of yamllint as it's fitting for our projects.

• Check all the rules it has and enable/configure them, as well as fix the issues it uncovers. When in doubt, discuss it with the team.
• Add the workflow to https://github.com/Lombiq/Hosting-Media-Theme too. - Not necessary, see: Configure YAML linting (OSOE-759) #658 (comment)
• For quotes, we'll need to use the following:

quoted-strings: {
    quote-type: single,
    required: only-when-needed
}

Jira issue

[DemeSzabolcs]

Regarding the rules I have one (currently), that I can't decide:

Should we allow implicit null values?

Example for implicit null: https://github.com/Lombiq/GitHub-Actions/blob/dev/.github/workflows/deploy-orchard1-to-azure-app-service.yml#L42C7-L43
Explicit would be: explicitly-null: null

[BenedekFarkas]

I would allow it and seeing the linked example, I'd rather enforce an explicit null in this case (if it plays nicely with GitHub Actions), because whitespace (or lack thereof) should not have syntactical meaning (unless https://en.wikipedia.org/wiki/Whitespace_%28programming_language%29 :D), but YAML is a language where indentation has syntactical meaning, so it's sort of a contradiction.

[Piedone]

Are there any similar examples? Because MAINTENANCE_USER_NAME and MAINTENANCE_PASSWORD should actually have at least a description like the others and mustn't be simply set to null. So, unless there's something markedly different then these, let's enforce explicit nulls and fix these by providing proper values.

[DemeSzabolcs]

So, unless there's something markedly different then these, let's enforce explicit nulls and fix these by providing proper values.

Well there is actually an interesting scenario, in case of

on:
  pull-request:
  push:
    branches:
      - dev

(https://github.com/Lombiq/GitHub-Actions/blob/dev/.github/workflows/validate-this-gha-refs.yml#L6)

The pull-request: line will trigger the warning. We can fix these too by adding null and it will still work, so:

on:
  pull-request: null
  push:
    branches:
      - dev

Do we want to use the rule still?

[Piedone]

Then not, but still fix the two maintenance secrets.

[DemeSzabolcs]

Add the workflow to https://github.com/Lombiq/Hosting-Media-Theme too.

Is this necessary? The workflow in OSOCE (https://github.com/Lombiq/Open-Source-Orchard-Core-Extensions/blob/dev/.github/workflows/build-and-test.yml#L73-L78) checks the submodules too. I already fixed the warnings in them.

[Piedone]

Then no, not necessary.