An interface for modules that are web apps. A web app here means that
the module is usable in a meaningful way from a normal web browser. The
web app should have a name and usually also a start page of some kind. In
addition, it may also have a page for each topic, however this isn't always
Returns the URL representing the topic with the given subject
identifier. If there is no meaningful page for the topic, return null.
This should not be dependent on whether the topic exists or not, the returned
page should be the URL if the topic is assumed to exist.