#75 add new email address for existing authors

[Rutger Vos]

BP writes:

> • When selecting an author for a citation, how can a user select an existing
> author yet also be able to update the author's email address? This action
> should edit the email for the new article, but leave the email for old one
> intact. (i.e. the email info should be stored in the author-citation joiner
> table)