Example App: add delete file example

[pjanik]

[#174012764]

@scytacki, the policy defined in functions codebase:
https://github.com/concord-consortium/token-service/blob/master/functions/src/base-resource-object.ts#L389-L418
was already listing delete actions, so no changes and new deployment were necessary there.

However, delete action wasn't working anyway. I had to update the policy attached to token-service-staging IAM role:
https://console.aws.amazon.com/iam/home?region=us-east-1#/roles/token-service-staging (ConcordQA)
using AWS UI. It fixed the delete action.

So, I went ahead and also did the same for production role:
https://console.aws.amazon.com/iam/home?region=us-east-1#/roles/token-service-production

If you open the demo page:
http://token-service.concord.org/branch/delete-file/example-app/index.html
you can create a new file, update, delete, and then update again (there's little delay between delete and update, it seems 404 is cached for a bit). So, that should cover everything that CFM needs.

I didn't work on the commit hash and file filters, as it's a bit separate task (no changes to functions this time), and I'd like to get CFM fixed asap. But I've added a story that I can come back to later: https://www.pivotaltracker.com/story/show/174197199

[scytacki]

Looks good to me.