As a sysadmin, I want to see similar insights to be grouped in the insight view

When multiple events that potentially generate a new insight, but that are ignored to avoid spamming the system with repeated insights/notifications (for instance, the RBL insights), the sysadmin should be informed about such repeated insights, in a way where they are grouped in a single one.

An example of how they are grouped can be see at the design at #218 (closed):

Requirements:

• For each insight type, similar insights generated in a row should be grouped in a single insight.
• The details of what "similar" means should be insight specific.
• Optionally I should be able to disable the grouping for all insights, through a option in the UI.