Add riskiness label when creating rollups

[rylev]

This change adds a riskiness label when creating rollups that uses the number of PRs and their rollup statuses to estimate riskiness. The current formula being used is (riskinessPoints + max(numPrs - 8, 0)) / 8 where riskinessPoints is the sum of all PRs where rollup=iffy equals 2 points, rollup=maybe equals 1 point and rollup=always equals 0 points.

Possible future improvements include:

• Asking for confirmation on rollups that are deemed very risky.
• Providing riskiness statistics in the actual rollup PR
• Tracking the merge success rate of risky rollups