A page that represents the leaf node in the UI hierarchy.
The direct parent of a WebUIPage is not specified as it can be either a WebUI or a WebUITab.
If the parent is a WebUI, the prefix is appended to the parent's address to form a full path.
Else, if the parent is a WebUITab, the prefix is appended to the super prefix of the parent
to form a relative path. The prefix must not contain slashes.