Use GlFormFields for admin_emails_form.vue

What does this MR do and why?

This MR replaces a "boring form" with our new fancy "gl-form-fields" gitlab-ui component: https://gitlab-org.gitlab.io/gitlab-ui/?path=/docs/base-form-form-fields--docs

In doing so, we manage to lose 75 lines of code 🎉

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Emailing users is a Premium feature. A developer EE licence is needed.
2. http://gdk.test:3000/admin/email
3. Check validation messages appears if you try and submit the form without completing any of the fields
4. Open dev tools and block gdk.test:3000/api/v4/groups.json?search=&per_page=20 and check an alert appears when trying to click the group/project dropdown
5. Fill out the form, submit and check network inspector to ensure the form data is posted as expected
6. Check sent e-mails in http://gdk.test:3000/rails/letter_opener

NOTE: If you need to re-submit the form in less than 10 minutes, run this from the rails console:

Gitlab::ExclusiveLease.cancel(Admin::EmailService::LEASE_KEY, Gitlab::ExclusiveLease.get_uuid(Admin::EmailService::LEASE_KEY))

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.