Display information on dataset health

[felixvanoost]

There is currently no easy way to see an overview of the 'health' of the activity dataset (e.g. how many activities are manual or are flagged) or how many activities contain extended sensor data (e.g. heart rate, cadence, or measured power). The tool should display the following information in a similar format to the summary statistics:

• Number of manual activities
• Number of flagged activities
• Number of activities with heart rate data
• Number of cycling activities with cadence data
• Number of cycling activities with measured power data
• Number of activities with temperature data