Event Form Validations

[YolineB]

PR Overview

Add validation for following fields in Event Form

• [x] Event Name
• [x] Event Start Date
• [x] Event Registration URL (update this to "Event URL")

Type of contribution

• [x] Code
• [ ] Documentation

Reference: Issue or Pull Request (PR)

• [x] Towards #284

Description of Changes

Update validation schema in EventForm.jsx, and update test for page accordingly

Before and After for UI Updates

Before:

After: Desktop

For PR Reviewer

• [ ] Does this file change the yarn.lock, package.json or package-lock.json file? If so, why?
• [ ] If this pr contains mobile and desktop changes, did you test on IphoneXr and desktop views?
• [ ] Does this file match the related tickets linked figma file, or does it pass the visual smell test?
• [ ] If this file contains javascript, does the javascript pass the smell test?
• [ ] If you don't feel super confident in your review, did you assign someone more senior to double check?