Fix add donation form not adding donations

[ethanhmaness]

Description

Add donation form was adding donors and donation items when appropriate, but not donations. This was because the form data was being validated, but the prevDonated field was never set. This has been fixed.

Relevant issue(s)

https://github.com/hack4impact-utk/Maintenance-Team/issues/32

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have assigned reviewers to this PR