Prevent invalid role assignment / selection

[timkinnane]

0.35.0

When creating a user, role select allows you to chose any role, regardless the scope. If you can chose Moderator or Owner which are room scoped roles, they don't get a base role of User and can't access any functionality, even to navigate to a room.

This role select helper needs to filter the role selection to only roles with global scope.
It should also only include Admin in the list if the current user can assign-admin-role.

We also have made changes to resolve #3656 (with help from Ethaan). He pointed out that there's no protection against changing the role of the last admin. All areas where admin can be un-assigned should invoke a helper to check you're not removing the last one - includes Permissions edit view, Remove Admin buttons, user role select from User Edit fly out.

[ggazzo]

fixed long time ago, any problem let me know