Project features does not validate EE features

In the ProjectFeature class, we define the FEATURES that the class is going to have. We have a validation that checks that we don't set any non-valid value in any of the features.

Nevertheless, in the ProjectFeature EE extension, we also define the EE_FEATURES, but we're not validating those because the validator uses directly the FEATURES constant.

We need to use the class available_features coming from the Featurable concern. That method returns all the features defined in both FOSS and EE.

As well, we can move the validator to the Featurable concern because it's a generic method. We can reuse it later when we implement group features.