Use OWNER access level for personal project owners

We give MAINTAINER access level to personal project owners:

> project.personal?
=> true
> project.team.human_max_access(project.owner.id)
=> "Maintainer"

In comparison, for a project in group:

> project.personal?
=> false
> project.team.human_max_access(user.id)
=> "Owner"

We currently have a workaround for this, so we end up with the expected behaviour: in ProjectPolicy we disregard their access level, and only check if they are Project#owner.

To simplify things, let's give owners the OWNER access level.