Improve db handling

See merge request !240

This will avoid some concurrency problem especially with SQLite.

Related to the use of the multiprocessing module.

See:
https://code.djangoproject.com/ticket/20562

This avoids database creation errors as other applications depends
on contenttypes.

Delete cached files

See merge request !239

This will go through all results and inputs of all experiments
that are using this cache.

Also remove the default delete action which should not be used
for the CachedFile objects.

Upgrade to django 1.11

See merge request !238

Currently the environment is "pre-created" on the runner
machine.

Otherwise the version from the CI are used and are too old

Otherwise system eggs for Django 1.9.5 are used on CI

This separates more cleanly the goal of each part. At the same
time the prefix path file can be kept in the sources rather than
being in /tmp that could be altered during a run.