Add support for the endpoint /challenge/{id}/addFileTasks, closes #83

[rYR79435]

This is my shot at implementing the endpoint I requested in #83.

I have used the swagger documentation for this.

Unfortuantely the API always returns maproulette.api.errors.InvalidJsonError: {'status': 400, 'message': 'Invalid JSON payload.'} no matter what I try. I have depleted my ideas for how to fix this, hence I'm creating this draft in hopes someone can help me.

Some notes:

• when I save my JSON to a file and then try to manually upload this via the MapRoulette UI (Edit Challenge -> Change Name or Data Source -> upload GeoJSON file), it works with no problems whatsoever (no errors, I can start tasks and they work as expected).
• I'm working with a Tag Fix challenge, my JSON is therefore in line-by-line format, as mentioned on learn.maproulette.org
• when I use the web-based API test on swagger, curl receives a Missing boundary header error. This is also true for curl on my local machine. I receive the same error whether I use the afforementioned JSON (that works in MR UI) or some (valid as well as invalid JSON) bogus data.

Here's a gist with example data: https://gist.github.com/rYR79435/08f2835ae1876c1e8c5ed8968cce208e

[rYR79435]

I figured out what the problem was. The API expects the file to be named 'json'.

I did so via this line in the MR frontend: https://github.com/osmlab/maproulette3/blob/e785c0e49af2b0bfcd9ffadbcbb199da6cca2ee5/src/services/Challenge/Challenge.js#L1020

[rYR79435]

Sure thing! Thank you for reviewing.