Add sourceURL to Bid and Tender

[georgiana-b]

We always wanted to have a reference to the original record (contract notice for tenders and contract award notice for bids). That's why we are storing the Tender.xTEDCNID and Bid.xTEDCANID. However there are plans to remove these fields as outlined in tenders-exposed#109).

For the manual data we will need to store the source URL of the article where the information was extracted from.
This gives us a great opportunity to also include the source URL of the original records for opentender data.