add media for challenge descriptions

[d3liri0us]

Add media files to challenge description by defining "media" key in challenge.yml and use /files/media/ path in description. (#209)

challenge-media.webm

[ColdHeat]

This is a good strategy but I think that in reality we need to be able to access the existing media config values.

The main issue is that we don't have a good way to self-reference the config.

Pages support a feature where you can replace like {{ logo }} with the media URL. Can you change the behavior in your PR to not have the media key in challenge.yml and instead inject with the {{ }} syntax?