feat(github): Allow to skip CI for some commits.

[lotas]

For a single commit push event with a message that contains "[skip ci]" or "[ci skip]" no tasks would be created.

Fixes #5311

[lotas]

This is currently implemented for the "push" events with a single commit in it only.

I'm not sure if we should also support pull requests with titles that includes "[ci skip]" keyword.

[petemoore]

I like it.
It occurs to me it might(?) already be possible to implement this in the .taskcluster.yml via json-e, if there is a way to check if the commit message contains the allowed skip terms via json-e, I guess it could return an empty list of tasks. But I've no idea how that would look in json-e and I think your solution is much easier for users.

One thing I'd like to call out is that in general, interpretation of commit messages is likely to be project-specific, so I think taskgraph is a better place to interpret directives in commit messages for anything else we add, but for the one and only case of skipping tasks, it indeed needs to be either json-e or the taskcluster github service that does this, since taskgraph requires that a task is already running.

[petemoore]

Love it, thanks for adding the tests too!