Allow multiple sources in manual entry

[sratcliffe118]

Canada data set has multiple sources that are linked to a single line list row. Need to support ability to link multiple sources to 1 line list entry.

There can be 1 primary source and multiple secondary sources

Sample set of Canada data:https://docs.google.com/spreadsheets/d/1VCKn28KbJ8-iiZ8ZFIQ8pPnBWefMoA2YMrxaZZDjQDo/edit#gid=0