Move related list partial to a component

This way the code is easier to follow; the code checking whether the
list has contents is in the partial rendering the list.

We also remove some duplication setting up related content in the
controllers.

For some reason, we have to manually ignore i18n keys which were
automatically ignored when the code was in the view.

app/controllers/proposals_controller.rb

@@ -29,7 +29,6 @@ class ProposalsController < ApplicationController
     super
     @notifications = @proposal.notifications
     @notifications = @proposal.notifications.not_moderated
-    @related_contents = Kaminari.paginate_array(@proposal.relationed_contents).page(params[:page]).per(5)
 
     if request.path != proposal_path(@proposal)
       redirect_to proposal_path(@proposal), status: :moved_permanently