Don't allow Guest users to move themselves into a paid seat
## Problem to solve
Currently, a Guest user can move *themselves* into a paid seat by creating a project in their personal namespace and adding themselves to a role higher than Guest on that project.
This means that admins have less visibility and control over whether or not a user takes up a paid seat.
https://docs.gitlab.com/ee/user/permissions.html#free-guest-users-ultimate
## Proposal
Guest users should either:
1. Not be able to create personal projects
2. Only have access to Free features on their personal projects
epic