Enhance the role based factor to have points per role

[brendanheywood]

Building on: https://github.com/catalyst/moodle-tool_mfa/issues/219

Being able to have points per role would mean you could have much more fine grained requirements, ie if you had extremely high mfa levels for privileged users who needed 2 factors.

The list of roles would be ordered by the least points, ie if you have 2 roles then it chooses the least points of the 2