Add support for multiple citations with a separate field for each citation.

Right now, students may cite multiple citations using commas to separate their citations.

This suggestion is to allow multiple citations to be added, each in their own citation field, to encourage multiple citations.

*Exciting note: The use of citations on Packback versus LMS discussion has been studied and confirmed in several independent studies, and on average students are approximately 3x more likely to cite a source on Packback versus LMS discussion boards.*