WIP: Convert existing Pages domain new/edit form from Haml to Vue

What does this MR do?

Converts the existing Pages domain new/edit form to Vue:

The new Vue component will make use of the existing Pages domains API: https://docs.gitlab.com/ee/api/pages_domains.html#update-pages-domain

What are the relevant issue numbers?

This MR is a first step towards a MVC related to https://gitlab.com/gitlab-org/gitlab-ce/issues/28996.

Once this component has been converted to Vue, the mockup in this comment can be implemented (which will happen in a separate MR): https://gitlab.com/gitlab-org/gitlab-ce/issues/28996#note_152137057

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer