Simplify rootUrl option

[wojtek-krysiak]

The problem

• there are 3 separate options in AdmnBroOptions for defining paths which is complicated
• they are not related to each other - changing root path doesn't change loginPath etc

The solution AC

• there should be one main option which will also setup login and logout routes
• login and logout routes could be optionally renamed (to discuss)
• everything should be described in the docs
• all the plugins should work after the update

Other approaches are possible - to discuss. The Goal is to simplify.