Add tasks runner script.

This script runs the deferred imports. It is meant to
be called from a cronjob at regular intervals – every
one or two minutes is probably a good setting.

It checks if another instance is already running before
doing its thing.

If anything goes wrong (any of the called processes exits
with non-zero condition) it will send an alert to GitLab,
provided that the authorisation key is known.

var/README.md

@@ -0,0 +1,3 @@
+# Varying data directory
+
+This is used for storage of temporary data files such as those produced by [`/bin/runner.sh`](../bin/runner.sh) and must therefore be writable by the Dougal user.