Add ability to sort and filter Segment Leaderboard on power and heart rate

[pbanders]

When viewing a segment leaderboard, enable clicking on a column heading to sort the displayed table. Clicking once would sort descending, a second time would sort ascending. Highly useful for reviewing your own segment history and sorting by average power or average heartrate. 

[Jane]

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[Supamark]

I'd like to see segment rankings based on average power, not time. Power is a much more equal comparison, since it is not influenced by (tail)wind or other circumstances.