Add new API call api/core/permissions/register

Allows applications to register new permissions that they use. This is
intended so that applications can have permissions they won't
necessarily know about before intialization (run-time permissions),
such as the ability to write to a specific forum. Applications are
restricted to registering applications within their own scope
(as enforced elsewhere in the permissions setup).

Signed-off-by: Tyler O'Meara <Tyler@TylerOMeara.com>