add usage count for subtabs in module

[ivokwee]

Currently the number of visits to the nav tabs are counted for each session. We would like to count the visits/clicks for the subtabs in each board module. This can be done with a similar reactive as input$nav but for each board. Implementation should be either inside the module, or on app level with appropriate namespaced inputs (e.g. input$load-navtabs??).

[ivokwee]

stale. closing.