Make it possible to upload challenge files

[Gitoffomalawn]

Currently, it's only possible to make challenges without providing the users with a file (such as a text file, or a pcap). This makes certain challenges difficult to implement without a dedicated secondary infrastructure.

Features that would solve this issue would be:

• Possibility for admin to upload files.
• Possibility for users to download files.

[rbclark]

Agreed that this would be a useful feature for us to have. The biggest issue is we normally use Heroku for deployment which does not provide any sort of permanent back-end storage. This means whatever file upload we end up using here should have the option of pointing at multiple back-ends, including S3 which is what we normally use.

[Bialogs]

I think we can wrap this up as a duplicate of #89 and make sure to use activestorage so people aren't forced into s3...