feat: real preview URL example

[KaelWD]

Changes

• Domain preview replaces {{pr_id}} and {{random}} not just {{domain}}
• Live updates
• {{random}} is now the same value for all URLs
• Refactored 4 different implementations to all use the same get_preview_fqdn function.

Remaining tasks:

• [ ] Make sure all the other generate_preview_fqdn methods still work correctly