Onboarding: Add role description to dropdown items

The UI enables us to add descriptions for roles, but they only show up when they are buttons, not the dropdown.

There is more than enough space on desktops at least (and can be handled via mobile) to show the role descriptions. 

I can understand why it's not been done in the outset, however via clever use of spacing (e.g. putting the description under the role name and making the item bigger) it can be properly handled.