Add ability to filter by role to organization memberships controlller

[joshsmith]

Right now missing ability to filter by role. This is currently an option for the "filter by organization slug" index endpoint on the Rails api:

    def filter_params
      filter_params = {}
      filter_params[:role] = params[:role] if params[:role]
      filter_params[:id] = id_params if params.fetch(:filter, {})[:id]
      filter_params
    end

For #26.