pc/modals: Validate phone and email even when they're optional

[ssangervasi]

Closes #2220

Code Changes

• Extracted Yup conditionals to a helper module.
• Gave the fields validation rules, but are explicitly optional when they should be.
• Cypress test.

Steps to Confirm

• "required" inputs should behave as before
• "optional" inputs should be require a valid format if they're filled, or be left blank.
  • The only invalid input that the phone field accepted seemed to be really long (>15 char) digits.

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation:
  • documentation complete, PR opened in fidesdocs
  • documentation issue created in fidesdocs
• Issue Requirements are Met
• Relevant Follow-Up Issues Created
• Update CHANGELOG.md

Description Of Changes

With the default config, phone is optional but it's been input with too many digits: