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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It's used here, that whole object is stored in the `user` property of the local session. (Line 34 below)