False examples for quality control

[xavierfav]

In order to test if a user can perform the annotation task and to filter spams, we sometimes add test examples in the validation form to see if the user is able to recognize the sound entity. If the user succeed, this information is stored with the votes (is_trustable field).
We need also to add some false examples in the validation form.

Work to be done:

• Annotate "relevant" false examples here
• Add a field false_examples in TaxonomyNode
• Add the false examples the same way positive examples are sometimes added in the validation form and check that the user answers correctly.