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/checks.py

@@ -84,3 +84,20 @@ def binaries_check(app_configs, **kwargs):
             check_messages.append(Warning(error.format(binary), hint))
 
     return check_messages
+
+
+@register()
+def config_check(app_configs, **kwargs):
+    warning = (
+        "It looks like you have PAPERLESS_SHARED_SECRET defined.  Note that "
+        "in the \npast, this variable was used for both API authentication "
+        "and as the mail \nkeyword.  As the API no no longer uses it, this "
+        "variable has been renamed to \nPAPERLESS_EMAIL_SECRET, so if you're "
+        "using the mail feature, you'd best update \nyour variable name.\n\n"
+        "The old variable will stop working in a few months."
+    )
+
+    if os.getenv("PAPERLESS_SHARED_SECRET"):
+        return [Warning(warning)]
+
+    return []