Add app health display in application list of web UI

This commits add a health summary in the application list component of the weg UI.
To do this two new attributes are added to the /v2/apps JSON:

  tasksHealthy, tasksUnhealthy

To calculate these two numbers efficiently, the TaskTracker is extended to store the health
value in addition to the MarathonTasks (encapsulated in MarathonTasksWithHealth). The
HealthCheckActor - which receives the health check results from the checks - stores
new health check results in the TaskTracker.
Add app health pie component