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 to use the same timestamp as the other rails logs and if it would be structured as json.

We should also change the timestamp so it is consistent with other log formats (discussion below). For reference here are the other 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