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/settings.py

@@ -61,6 +61,7 @@ INSTALLED_APPS = [
     "django_extensions",
 
     "documents.apps.DocumentsConfig",
+    "paperless_tesseract.apps.PaperlessTesseractConfig",
 
     "flat_responsive",
     "django.contrib.admin",
@@ -70,6 +71,9 @@ INSTALLED_APPS = [
 
 ]
 
+if os.getenv("PAPERLESS_INSTALLED_APPS"):
+    INSTALLED_APPS += os.getenv("PAPERLESS_INSTALLED_APPS").split(",")
+
 MIDDLEWARE_CLASSES = [
     'django.middleware.security.SecurityMiddleware',
     'django.contrib.sessions.middleware.SessionMiddleware',