Distinguish between paths and filepaths for solutions

[kytrinyx]

When submitting a solution we should ensure that the path that we send to the server is a normalized path relative to the exercise directory. This normalized path should always use forward slashes.

So for Hello World in Scala the path, when submitting from any operating system, would be:

src/main/scala/HelloWorld.scala

When downloading, we need to use Go's filepath.FromSlash to translate back to something that will work on the filesystem in question.

I will be writing a data transition on the backend to clean up all the existing filepaths.