Iss1678 - Creation of default inputs and PRTs update

[EJMFarrow]

• Only add default input if question text contains [[input:ans1]].
• Only add default PRT if questiontext or specificfeedback contains [[feedback:prt1]].
• Only add default specficfeedback ([[feedback:prt1]]) if questiontext contains [[input:ans1]].
• Set defaultmark to zero if no input.
• Set stackversion default to '' to avoid testing/maintenance issues.
• Apply to both API and XML import.
• Fix YML diff to cope with the changes.
• Add unit tests.
  Building STACK questions from templates and partial XML #1678

[sangwinc]

Thanks @EJMFarrow that looks good to me.