Drop SHARED_SECRET in favour of EMAIL_SECRET

Originally we used SHARED secret both for email and for the API.  That
was a bad idea, and now that we're only using this value for one case,
I've renamed it to reflect its actual use.

src/paperless/urls.py

@@ -34,18 +34,15 @@ urlpatterns = [
         name="fetch"
     ),
 
+    # File uploads
+    url(r"^push$", csrf_exempt(PushView.as_view()), name="push"),
+
     # The Django admin
     url(r"admin/", admin.site.urls),
     url(r"", admin.site.urls),  # This is going away
 
 ] + static.static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)
 
-if settings.SHARED_SECRET:
-    urlpatterns.insert(
-        0,
-        url(r"^push$", csrf_exempt(PushView.as_view()), name="push")
-    )
-
 # Text in each page's <h1> (and above login form).
 admin.site.site_header = 'Paperless'
 # Text at the end of each page's <title>.