feat: refactor for pluggable consumers

I've broken out the OCR-specific code from the consumers and dumped it
all into its own app, `paperless_tesseract`.  This new app should serve
as a sample of how to create one's own consumer for different file
types.

Documentation for how to do this isn't ready yet, but for the impatient:

* Create a new app
    * containing a `parsers.py` for your parser modelled after
      `paperless_tesseract.parsers.RasterisedDocumentParser`
    * containing a `signals.py` with a handler moddelled after
      `paperless_tesseract.signals.ConsumerDeclaration`
    * connect the signal handler to
      `documents.signals.document_consumer_declaration` in
      `your_app.apps`
* Install the app into Paperless by declaring
  `PAPERLESS_INSTALLED_APPS=your_app`.  Additional apps should be
  separated with commas.
* Restart the consumer

src/documents/signals/__init__.py

@@ -2,3 +2,4 @@ from django.dispatch import Signal
 
 document_consumption_started = Signal(providing_args=["filename"])
 document_consumption_finished = Signal(providing_args=["document"])
+document_consumer_declaration = Signal(providing_args=[])

src/documents/signals/handlers.py

@@ -1,6 +1,5 @@
 import logging
 import os
-
 from subprocess import Popen
 
 from django.conf import settings