Enabled strong_parameters across all models/controllers.

All models are now using ActiveModel::ForbiddenAttributesProtection, which shifts the responsibility for parameter whitelisting for mass-assignments from the model to the controller. attr_accessible has been disabled and removed as this functionality replaces that.

The require_parameters method in the ApplicationController has been removed in favor of strong_parameters' #require method.

It is important to note that there is still some refactoring required to get all parameters to pass through #require and #permit so that we can guarantee that parameter values are scalar. Currently strong_parameters, in most cases, is only being utilized to require parameters and to whitelist the few places that do mass-assignments.