Add the welcome bot

[GeorgianaElena]

The bot will welcome first time contributors (first issue, first opened PR, first merged PR per repository).

Example:

Welcome msg link	Congrats msg link

Todos left:

• Mention somewhere about the introduce-yourself thread on Discourse (relevant issue)
• Enable the bot for some/all repos

Closes jupyterhub/team-compass#298

[choldgraf]

looks great! ping when you think it's ready for a final review / merge

[GeorgianaElena]

Thanks @betatim 🎉

Before we can test the bot, we have to "activate" it from the org account.
Should we configure it for all the repos (and hope it uses a new enough probot version to actually work being configured from this central repo 🤞)?

Tim, do you have access to the org account to do this?

I think it will also be a good idea to extend the support bot to all the repos as well (the issue in team-compass has the link to the support bot app page).

[betatim]

I do have the permission and the welcome bot should now be active on all repositories in the jupyterhub organisation.

[choldgraf]

wohoo!