WebSearch: redirect merged records to the new URL

• Checks if the current record has been deleted and has been merged in websearch_interface.py, search_engine.py and webbasket.py.
• Creates function called 'get_merged_recid' in order to obtain from the subfield 970__d the record ID of the record with which the given record was merged.
• Builds the new URL with the merged record ID and redirect the deleted record URL to the new one.
• Calls this function in WebSearch, WebBasket and BibFormat.
• Adds in 'bibformat_utils.py' the marcxml out for the subfield 970__d.
• Removes the deleted record from its corresponding basket, keep its notes and adds the merged record in the corresponding basket with the notes kept before.
• Adds parameter 'date_creation' wherever it is needed in order to keep the dates in which the notes of the deleted record were created. (closes #514)