Handle rotation of JSON log files

We were not managing the /etc/logrotate.d/nginx file, so
were relying on the package default. This wasn't aware of our *.json
log files. Oops!
Switch to date-based log rotation

Considering these files can be quite large, and we are very much
interested in archiving them, make logrotate append a date-stamp
rather than a number.

This makes them work better with archival tools (such as duplicity),
and makes for less head scratching on later analysis.

In the process, make the duration we keep the logs on disk locally
configurable. 365 days is nice (and the previous default), but that
could chew up a lot of space.
Add nginx.readable_doc_dir_globs for apparmor

We are getting a lot of apparmor warnings when nginx is configured
to serve static assets.

This allows us to specify our static asset dirs, to silence apparmor
(and potentially turn on enforce mode)
update CHANGELOG
Fix ngnix worker log rotation - using wrong user

We use the 'nginx' user for nginx, rather than the package default of
'www-data'. This is likely a hangover from a migration from CentOS, but
now we're bedded in we may as well stick with it.

Ensure the log dir is readable/writable by nginx user. Previously it wasn't,
so workers were struggling to reread log files.
update CHANGELOG
Merge pull request #20 from ministryofjustice/logrotate_json

Handle rotation of JSON log files; change to daily, date-based rotation; Apparmor fix for static assets.