Add status about replication slots to Geo Primary status screen

Follow on to issue #3617 (closed)

Unused replication slots can cause large amounts of log data to build up. #3617 (closed) adds documentation, but we should add status information on the Geo primary status screen.

Something along the lines of current status items

Replication slots: 1/2 (50%)

Related Issue: #3195 (closed)