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

[peaklabs-dev]

@KaelWD are you still working on this?

[KaelWD]

I probably won't get around to finishing it this week, you're welcome to push to my branch though if you really want to get it in. I was kinda hoping my actually important changes would be reviewed before this one: #3845 + #4067