ability to customize package routes

[pxpm]

We got a request to customize the package routes in #17

It was kind of expected since we use named routes that can't be overwritten by registering developer routes before the package ones.

A good practice in this cases is to have a configuration that allow the developers to disable package route registering and they register the routes themselves.

I've implemented that in this PR, but I went a little bit deeper on the real issue here.

I think the fact that you are in https://something.com/my-application/dasboard and you change the language in https://something.com/set-locale/pt instead of https://something.com/my-application/set-local/pt is a little bit weird.

I introduced a new configuration and tweaked the route a little bit, so that if developer enable use_backpack_route_prefix_on_admin_routes, if the current route has the admin prefix, we also add the admin prefix in the "set locale route".

Let me know what you think @promatik

[promatik]

@pxpm added small changes to comments and variable names 👌

I agree with these changes, I think @tabacitu should also agree/give his opinion 😶 Since he was unwilling to add the config file at the beginning, but I don't see a way around 🤷‍♂️