Previously, the original source for a reference file was implicit:

1. If accession number starts with `LRG_`, it came from the LRG FTP
   archive.
2. If a download URL is known, it was downloaded from there.
3. If slice data is known, it was sliced from the NCBI.
4. If a GI number is known, it was downloaded from the NCBI.
5. Otherwise, it was uploaded.

In preparation for the removal of GI numbers (#349), this had to be
revisited. We now store the source explicitely in a new `source` field
on the `Reference` model. If additional information is needed to
re-fetch the file from this source (e.g., download URL), this is stored
in a new `source_data` field (always serialized as a string). This
scheme should be both more explicit and more generic.