Streamlining onboarding and offboarding checklists

FYI @brittanyr @sytses @jparrow

Fixes https://gitlab.com/gitlab-com/peopleops/issues/38

This is a rather large merge request, there kind of was no way not to make it a large merge request. How I made it was by having both the onboarding and offboarding checklists open next to each other in Atom, and going through the listed items one by one. In some cases, this resulted to adding topics to the onboarding (such as suggesting what email list to add the person to). In several cases, I found it to make more sense and easier reading/scrolling by grouping items related to a main topic. The grouping also allows for a different set of items to be listed in onboarding/offboarding. This is especially relevant for dealing with the email account and with 1password.

To review, I'd recommend checking out the branch locally, and stepping through the items one by one.

The alternative to doing this large merge request, is to simply add a task to the offboarding checklist that says "Review the onboarding checklist, and add relevant offboarding topics here". That would also be DRYer of course, but would lead to more time in setting up the offboarding checklist when it comes up.