privilege escalation possible #2105

Describe the bug
A non-admin user with the Manage users permission can obtain admin privileges by giving himself the admin role.

Steps To Reproduce
Steps to reproduce the behavior:

1. Add a custom role with the Manage users permission
2. Add a new user with this new role but without the admin role
3. Login as the new user
4. Go to your own Users settings
5. Give yourself the admin role
6. You should now have admin privileges

Expected behavior
It should not be possible to obtain more privileges than you already have.

Your Configuration (please complete the following information):

• Exact BookStack Version (Found in settings): v0.29.1
• Hosting Method: Docker