Autogreet new mentors

[rickr]

When a new mentor joins our mentor-internal channel they should be automatically messaged with a welcome message, a link to the mentorship repo and the guidebook.

[jjhampton]

I'd suggest the link be to the brief 'Intro to Mentoring' document on Discourse instead of the mentorship guidebook. The guidebook contains outdated and inaccurate information.

We also no longer have a mentorship repo.

[jjhampton]

I'm going to look at implementing this feature tomorrow. This will be a good step one in automating many of the tasks that our mentor volunteers currently do to coordinate/manage the mentorship program.

Related to Mentorship Management Improvements issue in main repo.