The notifications center works with gates (services)
and doors (services instances).
b42260e53626 introduced a notifications:payload command to
output a notification from a payload and those information.
To use this command with Phabricator too, we need to be able
to build the classes Phabricator want from only the instance
name, not from the full URL.
This refactoring allows to simplify constructors when handling
a Phabricator payload.