Fix issue with task shortcodes

Part of the code assumed shortcodes were globally unique, and another part
assumed the shortcodes are unique per project (the latter is correct).

Now the project ID is taken from the URL the Subversion hook pushes to.

docs/docs/user_manual/subversion.md

@@ -23,3 +23,6 @@ file is bundled with Attract's source code, and needs to be copied to the Subver
     }
 
 The authentication token must be created on the server using `manage.py attract create_svner_account {emailaddress} {project-url}`
+
+The project should have an `svn_url` extension property in the MongoDB database, which points to
+the Subversion server URL. This is for logging use only.