[Reviewer Andy] Don't crash if ENUM not enabled

Andy

Can you review my fix to the I-CSCF crash when ENUM isn't enabled.  I refactored the code slightly to move a chunk of processing in to the ICSCFSproutlet (from the ICSCFSproutletTsx) to neaten it up a bit.

All the UTs pass, but I haven't added a specific test case as it would be a chunk of work in the infrastructure.

Mike
This could do with a doxygen comment (I'm surprised that `translate_tel_uri` doesn't take a `tel: URI`).