Run tutorial attempts in a blank jail

[Matt Giuca]

Currently, the students tutorial attempts are run in their own jail. This
makes it possible for students to interact with their usual jail inside an
exercise. This means students may depend upon their private files,
destroying the accountability of the submitted exercises. It also means we
can't re-test solutions because it might mutate their jail.

Also, the sample solutions are currently run OUTSIDE any jail (in the
webserver). These need to be run inside a jail.

Create a blank jail on the fly for each attempt. Run as the user, but
inside this special temporary blank jail (per user). Run sample solutions
as a special persistent user created just for this purpose.

Note that this enables re-testing of student solutions when an exercise is
updated.