I fixed a couple of issues with the user management system

I fixed the following issues with the user management system:

* When you install Glav.in the first user is an `admin` and not an `owner`

* I could downgrade from owner to `contributor` by submiting a post request with `user_level = 2`

* `Admins` cannot edit themselves

* If I am an `admin` and change the `user_level` to -1 I appear as a `contributor` but I have `owner` privilages except I cannot edit myself

* If a user has `user_level = -1` he is able to delete the owner

* An `admin` or `owner` is able to create a user with `user_level = -1`