Make Kaffy integration more explicit

**Describe the problem you're proposing to solve**
Now we have to write a bunch of staff in various files. And it is not clear which URLs Kaffy uses, so putting it in scope with others things might break smth.

**Describe the solution you'd like**
Use the way of live dashboard does. So we have to do smth like this:

```elixir
defmodule MyAppWeb.Router do

import Kaffy.Router

scope "/admin" do
pipe_through :browser
kaffy "/", repo: MyApp.Repo
end
end
```