Simplify SystemHookUrlValidator to inherit from PublicUrlValidator
Refactor specs to move out shared examples to be used in both
system hooks and public url validators.

Updating security/webhooks.md to match new behaviour
as well as drying up few specs to extract shared
examples

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.

- The most common use case for qualified_domain_validator currently is
to allow blank ([]) but not allow nil. Modify the
qualified_domain_validator to support this use case.

- Validate that the entries contain no unicode, html tags and are not
larger than 255 characters.

Since we use `Namespace#path` to build routes and URLs we can lessen
the restrictions on `Namespace#name` so it can accomodate a user's name.

Also prevents ReDoS vulnerability

Renamed UrlValidator to AddressableUrlValidator to avoid 'url:' naming collision with ActiveModel::Validations::UrlValidator in 'validates' statement.
Make use of the options attribute of the parent class ActiveModel::EachValidator.
Add more options: allow_nil, allow_blank, message.
Renamed 'protocols' option to 'schemes' to match the option naming from UrlValidator.