Co-applicant functionality

[wes-otf]

In determining the full scope of the Partner role, has come up a few times that the ability to have "co-applicants" would be nice to have. @frjo suggested that both applicants and staff could assign the co-applicant and would provide the email of the intended co-applicant. In selecting, staff would receive a list of possible users & suggestions (similar to assigning a partner/reviewer) but the applicant would not to preserve the privacy of existing users in the system.

I imagine the user being assigned as co-applicant should have some option to actually accept being a co-applicant, along with the ability to remove themselves from the application if any applicant can do this assigning.

[wes-otf]

this is a huge "nice-to-have" - not a current priority or something that is being heavily requested