Added support for different colors of change-rate based on metric trend

It changes a color of the div.change-rate DOM element (and its children) based on metric trend:
- if the value increased and a thresholdComparator is set to "higherIsBetter" - indicator stays green
- if the value increased and a thresholdComparator is set to "lowerIsBetter" - indicator changes to red
- if the value decreased and a thresholdComparator is set to "higherIsBetter" - indicator changes to red
- if the value decreased and a thresholdComparator is set to "lowerIsBetter" - indicator stays green

Now, trend indicators (called "change-rates") are more verbose.