Ability to remove Gitlab Pages default URL if custom URL added.
### Problem to solve
After adding custom domain under GitLab Pages, allow the default URL (username.gitlab.io) to be removed.
### Further details
Avoids links to incorrect URL, or Google/web searches to Gitlab.io URL rather than custom domain.
### Proposal
Allow default username.gitlab.io URL to be deleted (if custom domain is connected)
### What does success look like, and how can we measure that?
It works...
### Links / references
issue