Add endpoint supporting arbitrary track images

[kytrinyx]

At the moment we've got broken images in the track documentation pages. We've been working around it by referencing the full path to the x-api endpoint that can serve them, which is kind of awful.

This lets us serve the track images directly from the exercism.io API. The next step is to use the new support for configuring the actual track documentation image paths when calling the trackler documentation function.

See:

• Serve files from tracks/:track_id/img path #3312
• Make documentation image paths configurable DEPRECATED.trackler#15

[kytrinyx]

After this is deployed, the URL http://exercism.io/api/v1/tracks/java/images/docs/img/eclipse-help-dropdown.png should return the image from the Java track.