[74] Enter Evaluators A11y Fix

[emmabjj]

Related ticket: #200 #74
Figma

Changes made in this PR:

• Given an existing user who was created without a first and last name,
  when adding that user as an evaluator to a challenge phase,
  then require and update the user with the provided first and last name.
• Update the UI to use a single field for the first & last name instead of two separate form fields.
• Update the UI to remove the subtitle below the form fields.
• Add inline error messages and use usa-alert for form validation.
• Update the challenge phase title.
• Make alt text for "Add Evaluator" decorative

---

Updated UI

Validation errors

Empty form

Only a first name

Missing last name

User with invalid role

---

Required first and last name for existing user

Existing user without first and last name updated with name entered

---

Mobile

---

WAVE