Add reminder when a PR is ready to be merged

[marchersimon]

When you approve a PR which can't yet be merged, because it was just opened a few hours ago, you prabably mark the notification as done. This either leads to forgetting about the PR and leaving it unmerged for a long time or merging it too early to prevent this problem. This could be solved by a notification from tldr-bot with these requirements:

• The PR has enough approvals and everyone, who requested changes has approved.
• The PR is at least 2 days old and has had no activity for the last 12 hours.
• If there are enough approvals, but not everyone who requested changes approved, set the minimum time to 4 days.