Identify and create organization bots for automating GitHub repository administration

This MDBF task should be the starting point for a discussion on what new technologies we can use to automate as much as possible the administration of our GitHub repositories.

The usage of GitHub automation bots is increasing a lot, new bots are being developed continuously and GitHub pushes lots of new features to facilitate this type of bots to automate workflows as much as possible.

Here's a list of potential bots that we could use in our oganization based on things that are currently performed manually:

1. CLA Bot for handling contributor agreements - DONE

2. Welcome Bot for sending a welcome message to first-time contributors
Potential example: https://github.com/jupyterhub/jupyterhub/pull/3307#issuecomment-746052441

3. Bot for managing stale Pull Requests
Pontential example: https://probot.github.io/apps/stale/

4. Bot for triaging tasks
There is a potential example for auto-assigning reviewers to Pull Requests, but I'm unsure whether it has all the features we desire: https://github.com/apps/auto-assign
A big counter-argument in the past for such a tool was that there needs to be a mapping ServerComponent::ResponsibleReviewer to make the bot work and this kind of mapping can help entities hunt down people highly specialized on some areas of the server (great point).
There might be solutions, or if there aren't, there might be in the future, to have a functioning bot that operates on a private list with this kind of mapping. This needs more investigation.

Feel free to add more items in this list and please do debate the usefulness of such tools.