Implement Filestack File Upload

[rootelement]

Tasks

1. Remove the file attachment UI on the challenge editor
2. Replace it with Filestack.
3. Update the challenge object to post an array of file URLs as attachments

[acshields]

@rootelement - Is Filestack implemented in the backend? I know that was in progress.

[rootelement]

Filestack itself is a frontend thing. The challenge api has been updated to no longer accept uploads, just URLs, then pull the files from the s3 bucket or URL of the attachment. So yes, what needs to be done on the backend is done and in dev.

[vikasrohit]

@rootelement @acshields we created separate issue as we didn't find this one when starting the work for the filestack implementation at UI.

[vikasrohit]

Closing this one in favour of keeping history for the work already done for #917 (comment) in November