Webhook for update triggering is not working

[Temtaime]

Try to generate webhook for a package in the settings.
It produces non-working URL.
Also it would be great if it produce a curl or wget example.

[AMDphreak]

This belongs in the registry, not dub itself.

Tracked as: dlang/dub-registry#614

After Enable/Regenerate Secret, the package page keeps ?secret=... on req.fullURL when building webhook URLs, which corrupts them (e.g. https://code.dlang.org:443?secret=SECRET/api/packages/...). A fix PR will target dlang/dub-registry.