Merge API : questionnaire//vars and questionnaire//variables

[BulotF]

/api/persistence/questionnaire/{id}/vars : Get the variables of a questionnaire (used by Public Enemy) and /api/persistence/questionnaire/{id}/variables : Get the variables of a questionnaire, used for pogues frontend

can be merged : Replacing externalQuestionnaireVariables.Variable by externalQuestionnaireVariables in Pogues UI is enough to use the API useful to Public Enemy.

An evolution of the first one would be to get the variables from the "dereferenced" questionnaire, but :

1. referenced questionnaires should not be host questionnaires too
2. If the become host too, their variables list should be the "dereferenced" questionnaire's one, as for Public Enemy.