modelviewer links for temp models based on timestamp

[bloer]

modelviewer URLs are of the form "explore/name/version/view". Temporary models fake this structure by using "explore/id/_/view". But internally modelviewer generates relative URLs using the name version syntax. Since the version for temp models is a lastmodified string, those links will be invalid if the temporary model is changed.

Instead modelviewer should check if the viewed model is temporary or not and use id/_ syntax. BUT we need to make sure that this won't cause a database hit every time an URL needs to be build...