Previously, the view function was responsible for doing a first pass of
the validations done for RealmPlayground. It is no longer true now. This
refactors do_add_realm_playground to check_add_realm_playground and make
it responsible for validating the playground fields and doing error
handling for the ValidationError raised.
Dropping support for url_prefix for RealmPlayground, the server now uses
url_template instead only for playground creation, retrieval and audit
logging upon removal.
This does the necessary handling so that url_template is expanded with
the extracted code.
Fixes#25723.
Signed-off-by: Zixuan James Li <p359101898@gmail.com>