Add commenting feature

Admins can internally comment submissions (OK, strange bug,
interesting, grading failure?, ...).
ES: refactor Job generation in search_jobs_not_done().

Create methods to decide what must happen to a Submission or to
a UserTest. The intent is to move all the similar logic in these
methods.
ES: add "submission_" prefix to some methods for consistency.
ES: finish refactoring Job generation.

All Job creation is factored through submission_get_jobs() and
user_test_get_jobs().

Also, fix some logging and reimplement lowered priority for Jobs
that have already failed.
A lot of full stops.
Another docstring.
Use EXTRA_LOW priority for datasets that are not active.
Fix docstrings.
Remove a long-living connection when precaching files in Worker.
Worker: during precaching, download files only if they're not available.
Include isolate as a git submodule

Make the subdirectory isolate/ become a git submodule for the repository
cms-dev/isolate. We use the https:// address (instead of the git:// one)
to avoid authentication and permission issues. Also explain how to clone
in the README.

Fixes #1.
Fix compilation with isolate submodule.