Allow some ajax external functions to be called without a session

There are some external functions that return static data that is completely public. Examples are language strings and templates. If we create a second entry point for these scripts, and introduce a new field to the services.php array to mark the functions that are safe to call this way, we can improve performance. This new script would not lock the session, so it could handle parallel requests - and it would be able to verify whether or not to allow calling this external function in this way.

An alternative design that was discussed was to have a second ajax.php file in db/ for each component and bypass the webservices for these ajax functions - but I (Damo) would like to encourage/test the webservices framework alongside the ajax external functions.