We're using `eq` and `match_array` in most places, but there were a few
places where we were still checking each element is included in the
array. This is a bit dangerous, because the array could have duplicate
elements, and we wouldn't detect them with `include`.
So now we test in depth at the model level, and can be a bit more
relaxed about integration tests for translations.
Note we're defining some extra factories to make sure all translatable
attributes with presence validation rules are mandatory. This way we can
simplify the way we obtain required fields, using `required_attribute?`.
Otherwise, fields having an `unless` condition in their presence
validation rules would count as mandatory even when they're not.
