Simplify SystemHookUrlValidator to inherit from PublicUrlValidator
Refactor specs to move out shared examples to be used in both
system hooks and public url validators.
This MR adds new application setting to network section
`allow_local_requests_from_system_hooks`. Prior to this change
system hooks were allowed to do local network requests by default
and we are adding an ability for admins to control it.
Reuben Pereira