feature #907 sprint4 :
* by default, a suggestion has no validation object set. This property is set only when requested (during the first validation of the suggestion). This property can be
then updated for further validation (after a refusal). This characteristic is taken into account in the tests.
* update the signature of SuggestionBox$Suggestions#validate: it accepts now the ContributionValidation instance as a second argument. This one gives the information
about the validation to perform.
* change the property name 'state' in Suggestion by 'status'; 'state' is a reserved SQL-keyword, so the renaming is for avoiding some unexpected problems with
the persistence of suggestions. Tests are updated to take into account this change as well the SQL instructions to create the tables for the Suggestion Box application.