Add support for tracking laps

[dpfens]

The a user provides a lap distance and lap units, add a Lap button to track laps. Laps are a more specialized lap, in that laps have a constant distance, where splits can be taken at any distance interval. Laps track the time since the last lap, and splits track the time since the last split of any kind. This gives the user fast multi-tiered tracking of users and matches the existing mental model for tracking competitors in a race.