Add CORS headers to api requests

[jessicaaustin]

Description

Add the following headers to API server requests:

• Access-Control-Allow-Headers:Accept, Authorization, Content-Type, Origin
• Access-Control-Allow-Methods:GET, OPTIONS
• Access-Control-Allow-Origin:*

Motivation and Context

This allows an external application to request data from the api via Javascript. In our specific case, we request JSON data from /api/graph and display it in an internal ops dashboard.

Have you tested this? If so, how?

Ran the server locally and verified that the desired headers show up. Tested out with an AngularJS application that pulls in luigi task data.

Simple test that you can run in a dev console (requires jQuery): $.get("http://localhost:8082/api/graph", function(data) { console.dir(data.response); })

[mention-bot]

@jessicaaustin, thanks for your PR! By analyzing the annotation information on this pull request, we identified @ystopia, @erikbern and @freider to be potential reviewers

[erikbern]

lgtm!

[Tarrasch]

LGTM. Can you also accompany this patch with test cases?

[jessicaaustin]

@Tarrasch test added

successful run: https://travis-ci.org/spotify/luigi/jobs/164703448#L2821

[Tarrasch]

Awesome!

Is there any chance of open sourcing any of your dashboards? Or at least show screen-shots?

[jessicaaustin]

Yep, definitely. I'll post some screenshots at least, but there's a chance at least part of the dashboard will be publicly available. Thanks for merging this PR!