Fetching the user session from the backend on page load and purging

Fetching the user session from the backend on page load and purging it in the backend when logging out.

Depends on https://github.com/my-koop/module.user/pull/46 and addresses https://github.com/my-koop/service.website/issues/272

Until a system like the one proposed in https://github.com/my-koop/service.website/issues/185 is put in place, the NavBar will have to do the work of logging out the user itself.

Until an eventual front-end module bootstrap API is put in place, the AppWrapper will have to do the work of fetching the current user session itself.

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I went with a check on `actions.user` alone, we really want to avoid the case where the user module wouldn't be loaded, but if it is we assume the rest is gonna be available. (And if it's not it SHOULD break anyway as this would be pretty critical and require immediate attention)