Mark disabled pages domain for removal

What does this MR do?

As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/44696, we have many disabled pages domains which are being constantly verified.

This MR adds column remove_at to pages domain and set it to 1.week.from_now when domain being disabled or unsuccessfully verified first time.

Some spec refactoring is extracted in separate MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26178/diffs Actual domain removal also will be done in separate MR

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/44696

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