Add `source_url` to package `extras` field for the scraped datasets

[nightsh]

ref. #25

Tasks:

• [x] Add the source_url property to the resulting datasets in the data.json output
• [x] Make sure the source_url property passes validation and gets stored as an extras item in CKAN
• [x] Adjust the ckanext-ed package template so it can display where that dataset was scraped from

Acceptance criteria:

• [x] Scraped datasets in CKAN have a new item in the metadata table to show where they came from
• [x] ~Datasets without source_url that were harvested will have a link to the harvester source info page~
• [x] Datasets that were manually added will not have the extra property

[nightsh]

ETA for the remaining work: 2h