Initial Taskcluster config that successfully runs npm run lint

[biancadanforth]

v1 References:

• Example of a simple v1 (courtesy of owlish).

v0 References:

• Docs that include some helpful info on how to translate v0 into v1 (courtesy of owlish).
• Example of v0 that has a bunch tasks for linting testing etc. (courtesy of owlish).
• Normandy .taskcluster.yml with a decision task Python script that generates more tasks to occur in parallel (courtesy of mythmon).

[biancadanforth]

I merged this to verify that my changes to the 'github-push' task work correctly. This task should only run when pushing to the master branch.

Also, since an earlier commit in this PR triggered a TC task on all pushes and a later commit triggered that same TC task only on pushes to master, this PR was also merged to get rid of GitHub's PR state which was still seeking both the PR and push tasks to run and pass. (Note: this did not work in a subsequent PR, but I filed Bug 1514914 to fix it upstream).

BEFORE:

# When a commit is pushed, perform this task
  - $if: 'tasks_for == "github-push"'
    then:
      taskId: {$eval: as_slugid("push_task")}
      # ...

AFTER:

# When a commit is pushed to master, perform this task
# If a branch is merged without being updated first, the tests could pass on the branch
# but not when things are merged into master.
  - $if: 'tasks_for == "github-push"'
    then:
      $if: 'event.ref == "refs/heads/master"'
      then:
        taskId: {$eval: as_slugid("push_task")}
        # ...