Don't hide records during a system test

As mentioned in commits like a586ba806, a7664ad81, 006128da5, b41fbfa52
and c480cdd91, accessing the database after starting the browser with
the `visit` method sometimes results in database corruption and failing
tests on our CI due to the process running the test accessing the
database after the process running the browser has started.

In this case, we were hiding a proposal after starting the process
running the browser to check what happens when accessing a notification
for a hidden proposal. We can avoid database access in the middle of the
test by hidding a proposal before starting the browser. The process to
create a notification using the browser is already tested in other
specs, so we don't need to do it here as well.

Note that, to simplify the test, we're extracting the `notify_users`
method. I wonder whether this method should be called in an
`after_create` callback instead... That's a topic for another time,
though.

app/controllers/proposal_notifications_controller.rb

@@ -11,9 +11,8 @@ class ProposalNotificationsController < ApplicationController
     @notification = ProposalNotification.new(proposal_notification_params)
     @proposal = Proposal.find(proposal_notification_params[:proposal_id])
     if @notification.save
-      @proposal.users_to_notify.each do |user|
-        Notification.add(user, @notification)
-      end
+      @proposal.notify_users(@notification)
+
       redirect_to @notification, notice: I18n.t("flash.actions.create.proposal_notification")
     else
       render :new