Discourse stopped using PostUpload in https://github.com/discourse/discourse/commit/6765cf0fbc92ff24543e99e7c56f3f79091821e5. Since then, these importers have been writing to the table, but any data was totally unused. This commit updates the easy cases to use UploadReference, and adds an error to the discourse_merger import script, which needs more significant work.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
