Make it possible to turn off metric targets and thus make metrics "informational"

[fniessink]

Tasks:

• UI:
  • Add a "evaluate targets" yes/no field in the frontend
  • If a metric is informational, the status is always "informative"
  • In the metrics table, if a metric is informative:
    • use a blue (i) icon
    • leave the target column empty
    • use a light blue background color
  • Show informative status in dashboard
  • Show status in trend graph
  • Make sure informative metrics are hidden when showing metrics requiring action
  • Check sorting by status
• In shared_python, wherever the status is determined, prepare for informative status (model code).
• Do not expect the metric.informative field to be present (in other words, we won't write migration code for this feature).
• Update Quality-time source in data model
• Add feature tests
  • An informative metric with accepted technical debt still has status informative
• Update user docs
• Update screenshots
• Update changelog

Double check:

• External server does not need to be changed
• Internal server does not need to be changed
• Notifier does not need to be changed
• Collector does not need to be changed

Out of scope:

• Layout of the metric configuration tab