feat: Add Sign-out functionality (#46)

#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:
![2022-01-21 18 09 14](https://user-images.githubusercontent.com/88213859/150620847-94e4d22f-1dcf-451e-8b4a-cec24702ea6c.gif)

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.

codersdk/users_test.go

@@ -47,4 +47,12 @@ func TestUsers(t *testing.T) {
 		require.NoError(t, err)
 		require.Len(t, orgs, 1)
 	})
+
+	t.Run("LogoutIsSuccessful", func(t *testing.T) {
+		t.Parallel()
+		server := coderdtest.New(t)
+		_ = server.RandomInitialUser(t)
+		err := server.Client.Logout(context.Background())
+		require.NoError(t, err)
+	})
 }