add jobs.perl.org-jobs-counter as a sub module rather than duplicate it

[perigrin]

So looking at briandfoy/jobs.perl.org-jobs-counter, it seems it's already:

1. updated regularly
2. containing a full export of job_postings

This means rather than us duplicating everything (including our own cache of the job posts), we could just add it as a sub module and leverage the work that is already there.