Add flakiness labels based on flakiness reports count

What does this MR do and why?

This implements #61 (closed).

Every time a flakiness report is added, a flakiness label is set based on the number of occurrences reported:

• more than 1000 reports => flakiness::1
• between 500 and 999 => flakiness::2
• between 10 and 499 => flakiness::3
• less than 10 => flakiness::4