Backend changes to enable Oauth from angular ui

Summary:
- Add CORS headers to on /oauth
- Add CORS headers to API controller
- Handle OPTIONS request to api controllers, skip oauth token checks for this
- Add CORS headers to HTTP exceptions
- Handle OPTIONS requests to /oauth
- Decode json sent to oauth/token

Test Plan:
- Check for CORS headers on /oauth/token
- Check for CORS headers on /api/v2/posts
- Check for CORS headers on error responses (ie. api request with no token)

Reviewers: shadowhand

Reviewed By: shadowhand

Subscribers: spaudanjo

Differential Revision: https://phabricator.ushahidi.com/D337