Control which new/changed files appear

[JonJagger]

When you press [test] all your files are send to the runner service. It creates a new container and tar-pipes the files into it. It then runs cyber-dojo.sh (with a timeout). It then gathers stdout/stderr/status and determines the traffic-light colour.
It also returns new/changed text files. This is to support Approval Style test frameworks. However, some start-points have coverage, and these also generate text files. Eg gcovr generates .gcno files.
We need an allow_list, eg specifying file extensions, which control which generated/changed text files coming back from the runner are actually updated in the browser.

[JonJagger]

This should be specified in the start-point manifest.
It should happen in the browser.
It should be editable in the browser.