Implement json logging for application.log

The application.log is causing us difficulty (https://gitlab.com/gitlab-com/infrastructure/issues/4244#note_81693147). It would be helpful if it were structured as JSON and used the same timestamp format as other logs. For reference here are some other other log timestamp formats:

• Gitaly 2018-05-23T13:41:10Z
• Pages 2018-05-23T11:14:50Z
• production_json 2018-05-23T15:24:23.915Z
• geo 2018-05-23T16:04:23.624Z
• sidekiq: 2018-05-23T14:40:00.808Z
• api_json: 2018-05-23T15:47:07.471Z
• gitlab-shell: 2018-05-23T15:45:56+00:00
• application.log May 23, 2018 09:56

Proposal

Introduce a structured version of application.log:

• Retain application.log.
• Introduce a new structured logfile, application_json.log.
• In the structured logfile, use a timestamp format that is consistent with other log files, e.g. 2018-05-23T15:24:23.915Z. This is being worked on in another issue.