refactor: use PushEvent in global openrank

[frank-zsy]

In this PR, I modify the global OpenRank algorithm for a little bit.

• Use PushEvent instead of PullRequest merge event, because actually the pull request merge event will lead to a corresponding push event, but direct push will not contain a pull request event. Although we would like to encourage issue and pull request collaboration, direct pushes are also widely occurs in the open source world even in many really important projects. To fit the different collaboration model in open source world, use PushEvent with a relatively low weight will do.
• Set the monthly activity limit for a single developer to the 75th percentile of the overall active users, which is approximately achieving 2 PRs per month to reach the limit for that month.
• Output the users who active in more 300 repos within a month to help us to find out robot accounts.
• If calculation for a month fails, we should restart the process because graph database may not stable.