Add a Make Valid Handle Function for Custom Domain Handles

[RetroSunstar]

Prevents the user from putting invalid domain characters in the Change Handle field for Custom Domains, mentioned in #1528

Hopefully this is okay? 🩵🩷🩵🩷

[mary-ext]

why not check if the handle is actually valid rather than stripping invalid characters?
/[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*(?:\.[a-zA-Z]{2,})/

[RetroSunstar]

why not check if the handle is actually valid rather than stripping invalid characters?

Simply because that's how Bluesky handles it for .bsky.social handles, with the makeValidHandle function just above in handles.ts

I just put some characters that couldn't be used in a domain to begin with, esspecially the @ symbol as that's the subject of the original issue

Anything else I assume would be handled when the user goes to verify the DNS record or Text file? As the domain simply wouldn't exist

Alternatively, there's allowing the user to input these characters but show an error as soon as one is, which might affect typing less and allow the user to simply go back and change it? 🩵🩷🩵🩷