Add domain verification operation views

app/models/pages_domain.rb

app/presenters/pages_domain_presenter.rb

app/views/projects/pages_domains/_dns.html.haml

doc/user/enterprise_user/index.md

doc/user/img/get_domain_verification_code_v16_0.png

doc/user/img/retry_domain_verification_v16_0.png

ee/app/assets/javascripts/domain_verification/index.js

...scripts/pages/groups/settings/domain_verification/form.js

.../pages/groups/settings/domain_verification/index/index.js

...ts/pages/groups/settings/domain_verification/new/index.js

...s/pages/groups/settings/domain_verification/show/index.js

ee/app/helpers/groups/domain_verification_helper.rb

...roups/settings/domain_verification/_certificate.html.haml

.../views/groups/settings/domain_verification/_dns.html.haml

...views/groups/settings/domain_verification/_form.html.haml

...roups/settings/domain_verification/_helper_text.html.haml

...tings/domain_verification/_lets_encrypt_callout.html.haml

...views/groups/settings/domain_verification/index.html.haml

...p/views/groups/settings/domain_verification/new.html.haml

.../views/groups/settings/domain_verification/show.html.haml