Add onboarding task item to alert the buddy that they've been chosen

After being at GitLab for a bit and hearing other's experiences, it seems that it is very common for the chosen buddy to be unaware that they've been chosen. I propose that an extra task item be added to send a separate, personal email to inform them of their new buddy.

I think it is possible that due to various notification settings or email filters, the buddy may miss the email from being mentioned in the onboarding issue. A separate email directly to them is unlikely to be filtered or missed, leading to a better onboarding and buddy experience.