Refactor to remove trailHeadMarkers array

[anselmbradford]

Currently, there is a trailHeadMarkers array in the controller, which is used to store references to the trailheads marker is used to find the correct marker when the trailhead model modified. Ideally, the trailhead markers would be able to be associated with the model directly. Without a direct association, the trailHeadMarkers array is looped through to find the trailhead marker for a model in $scope.$watch('selectedTrailHead'. It's also used in the moveMarkerToMap and moveMarkerToCluster, which are used to move a marker to and from a marker cluster so that the selected trailhead doesn't get combined with a cluster when zooming out. These are both expensive methods that would benefit from this refactoring.