User should see alert if passwords don't match at signup

Describe the Issue
Currently, a guest will enter a username, password, and password again for verification. If they enter an invalid username or password, they will see an alert. But if their password and password verification don't match, they won't receive an alert. Nothing will add to the json, but the user also doesn't know if their account was saved - the fields just go blank.

Expected behavior
When a user is signing up, they should see an alert saying that their passwords don't match if that is true. This would pop up after they click "submit".

To Reproduce
1. Start the app using npm run start
2. Go to localhost:3000
3. Select the signup tab... Enter an 8+ character username and an 8+ character password
4. For the "verify password" field, enter a password different from the first password entry.
5. View the blank fields with no alert - here is the issue.