Deprecate source_url in favour of item_url

[axfelix]

Currently, we use a "source_url" field as a de facto item URL for export, and refer to it internally as "source_url" throughout. This is a legacy of beginning with oai_dc, in which item URLs were mostly frequently found in dc:source fields (even though this is arguably a violation of the DC guidelines, which specify that source should refer back to eg source material for the content, like a related item field, repository OAI implementations seem to contravene this across the board).

Now that we have a more consistent way of constructing item_url, we should probably deprecate this more fully. It's only used in a couple places in the exporter, where it is pulled to act as the canonical record source URL -- I think item_url could be a drop-in replacement here:

frdr_harvest/harvester/Exporter.py

Line 50 in b4ce527

records_sql = """SELECT recs.record_id, recs.title, recs.title_fr, recs.pub_date, recs.series, recs.source_url,

frdr_harvest/harvester/Exporter.py

Line 82 in b4ce527

['record_id', 'title', 'title_fr', 'pub_date', 'series', 'source_url', 'deleted', 'local_identifier',

It's used quite a bit more in the db interface code, mostly to pull dc:source: https://github.com/axfelix/frdr_harvest/blob/43bad15150241dcdc92615e332aa06bee073d011/harvester/DBInterface.py. These would take a little more care to clean up, making sure we keep using them as a fallback in some cases -- I'm not sure if we need to deprecate this field entirely, in case some repository is found to use it to link related identifiers, and ultimately I think we could probably just keep them and switch to defaulting to item_url in the exporter.