Add ability for challenge and hint descriptions to reference media files

[ColdHeat]

• Add ability for challenge and hint descriptions to reference media files

I felt this made more sense than adding media directly to the challenge.yml.

Thoughts @d3liri0us?

Related to #215

[d3liri0us]

Yeah, this makes sense.

My idea was that challenge repos should contain all the files related to the challenge. If all the media files are at one place, it might be confusing as to what belongs to pages and challenges. And also, it adds an extra layer of running ctf media to install challenges regardless of whether the media files are in their individual directories or one directory.