WIP: Logging configuration

[ovv]

Putting it out here to get feedback on the approach.

I figured adding a file to configure the whole of logging would be better than individual option (See #495 ).
I'm not sure how to best name the file, creating a paperless directory and simply naming it logging.yml seems fine but it changes where the configuration should be located.

I still need to do some more tests and write some documentation

[danielquinn]

This is all good stuff, and I just have a few notes:

1. I like the move to use YAML for configuration. Especially in the case of logging, this is a natural choice. My only concern though is that we're introducing another config file in another format, which isn't very user-friendly. Paperless has always followed the 12-factor methodology (configuration only through environment variables) to keep things portable, and adding this new file (and format) introduces a requirement for the existence of a readable file. If we're going to introduce this requirement, we should be conscious of the limitations it'll introduce in terms of portability between setups (docker vs bare metal vs some other implementation).
2. The use of Sentry is a great idea, but it's probably best to make use of their raven library rather than rolling our own integration. Sentry has some pretty good docs on this subject, but the TL;DR is that you need to add their library to INSTALLED_APPS which could be done in settings.py dynamically after detecting the preferred configuration.

[danielquinn]

Further to 1️⃣ above, would it be altogether crazy to set an environment variable to a JSON value that would represent the logging config? It'd be ugly, but at least more portable. Something like:

PAPERLESS_LOGGING={"version": 1, "disable_existing_loggers": false, "handlers": {"consumer": {"class": "documents.loggers.PaperlessLogger"}}, "loggers": {"documents": {"handlers": ["consumer"], "level": "INFO"}}}