Add a user to process logs for transition

We're setting this up initially to process the Fastly logs for Bouncer
and Redirector. The user needs:

  - to be able to read the logfiles in /srv/logs/log-1/cdn and
    write to this directory to mark logfiles as processed
  - an SSH key which we will add as a deploy key to the repos
    which it needs pull/push access to
  - a global .gitconfig with name and email address for committing
    processed data to these repos
Update group and permissions for Fastly logs directory

This is for consistency with the logfiles in that directory.
Add script to process Fastly logs

Clone/pull repos, process Fastly logs, commit hits files on a branch
in transition-stats, push.
Merge pull request #111 from alphagov/process_redirector_fastly_logs

Process Bouncer/Redirector Fastly logs