#37 implemented the Sign-_in_ flow, but there wasn't a Sign-_out_ flow as part of that PR (aside from letting the cookie expire... or manually deleting the cookie...), which is obviously not ideal.
This PR implements a basic sign-out flow, along with a very simple user dropdown:
Bringing in a few pruned down components for the `<UserDropdown />` to integrate into the `<NavBar />`.
In addition, this also implements a simple back-end API for `/logout` which just clears the session token.
Bryan
