Bugfix: Feeds total notifications

On this MR we did an improvement of only loading/processing 3 notifications per channel to the templates: !90 (merged)

However we need the total notifications per channel, for this part https://gitlab.cern.ch/push-notifications/notifications-consumer/-/blob/master/notifications_consumer/processors/email_feed/templates/feed_notification.html#L43

Adapt templates's payload, and the template itself to fix this problem.

Suggestion: change current payload from

channel_notifications = {
  channel_id1 = [notification1, notification2, notification3],
  channel_id2 = [notification4, notification5]
}

to

channel_notifications = {
  channel_id1 = {
    notifications: [notification1, notification2, notification3]
    total: 10
  },
  channel_id2 = {
    notifications: [notification4, notification5]
    total: 2
  }
}