Do not trigger builds on pull requests unless explicitly requested by committers

[hcho3]

See dmlc/xgboost#5176. Right now, every pull request (and edits made to them) will trigger a build in the CI server. Many pull requests are Work In Progress (WIP) and require further edits before a maintainer can merge them. To save cloud costs, no build should run on pull requests unless explicitly requested by a maintainer.

• Write a Probot app to look for keyword "ok to test" from a maintainer. There should be a database to store the list of pull requests marked as "okay to test"
• Modify Jenkins runner to query the database mentioned above.

[hcho3]

With dmlc/xgboost#5884, it should now be easier to do this.

[hcho3]

Fixed in dmlc/xgboost#8142