Implement features score thresholds

[macbre]

Render features widgets with a proper "priority" icon based on their scores and configured thresholds - see https://wikia.github.io/Mike/ for examples.

If these are not provided, render all metrics with a green icon.

thresholds:
  mayor: 50000  # features with scores greater than this value will have a yellow "card-header-warning" icon
  critical: 200000  # features with scores greater than this value will have a red "card-header-danger" icon