livesocket instance: `navigation`.

Navigation is an object that can define two functions: `beforeEach` and
`afterEach`. The `beforeEach` function is called before each navigation
event and can for example be used to store custom scroll positions. If
the `beforeEach` function returns `false`, the navigation will be aborted.
This can be used to implement a warning when the user tries to navigate
while a form is still being edited. The `afterEach` function is called
after a navigation event has been completed. It can be used to restore
the scroll position that was stored in the `beforeEach` function.