Consolidate messages and fix dismiss title

1. Dismiss title should be based on $event->getCategory() instead of $event->getType()
2. Remove various echo-email-batch-category-header-* messages
3. update echo-dismiss-title-* to echo-category-title-*

Change-Id: I02fc85072f3d5967668c94eb28c8ecff606023d0
Merge "Consolidate messages and fix dismiss title"