Registration Flow - Don't pre-select role

What does this MR do and why?

changes for issue #341852 (closed)

1. Adds front-end validation to require role
2. Adds error message for case when the user attempts to submit without a populated role
3. Adds empty state guidance messaging

Screenshots or screen recordings

gitlab-registration

How to set up and validate locally

1. Sign up as new user and sign in

2. On the /users/sign_up/welcome page, the role drop down will be defaulted to empty with message Select a role

3. When we try to submit the form on /users/sign_up/welcome page without a role, a frontend validation error will be thrown with message A role is required. Choose one that best describes what you do.

MR acceptance checklist

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