refactor: break document-only stuff into the paperless app

The `SessionOrBasicAuthMixin` and `StandardPagination` classes were
living in the documents app and I needed them in the new `reminders`
app, so this commit breaks them out of `documents` and puts them in the
central `paperless` app instead.

src/paperless/views.py

@@ -0,0 +1,7 @@
+from rest_framework.pagination import PageNumberPagination
+
+
+class StandardPagination(PageNumberPagination):
+    page_size = 25
+    page_size_query_param = "page-size"
+    max_page_size = 100000