UI - System Event Log

In Neo we're introducing a system event log to record events that happen across the cluster, see the parent MB, https://issues.couchbase.com/browse/MB-46215 for more details. 

There exists a ‘User log’ as part of NS Server which has some event type information and is shown in the UI, but it has many issues that do not currently make it appropriate to fill the need here.  For example, it has a filter option, but it’s really just a full text search of the data as it's not in a structured format. 

This new feature will replace the User Log that the UI is currently displaying with structured JSON data.  As part of that improvement, the UI should be filterable by the different JSON fields.