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/documents/forms.py

@@ -13,7 +13,6 @@ from .consumer import Consumer
 
 class UploadForm(forms.Form):
 
-    SECRET = settings.SHARED_SECRET
     TYPE_LOOKUP = {
         "application/pdf": Document.TYPE_PDF,
         "image/png": Document.TYPE_PNG,