Redirect to unique domain

What does this MR do?

When a GitLab Pages site has the unique domain enabled/available (Unique pages site domain (&9347 - closed)), GitLab Pages will redirect the standard namespace domain to the site unique domain. This will enable users to keep using saved URLs.

FYI: The whole unique domain feature is behind a feature flag

• Related to Redirect namespace domain to unique domain (#831 - closed)

Screen_Recording_2023-03-20_at_20.04.47
disclaimer for the demo above work instantly like that I disabled the Gitlab caching.

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines