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/paperless_tesseract/apps.py

@@ -0,0 +1,16 @@
+from django.apps import AppConfig
+
+
+class PaperlessTesseractConfig(AppConfig):
+
+    name = "paperless_tesseract"
+
+    def ready(self):
+
+        from documents.signals import document_consumer_declaration
+
+        from .signals import ConsumerDeclaration
+
+        document_consumer_declaration.connect(ConsumerDeclaration.handle)
+
+        AppConfig.ready(self)