Support a basic client-side login flow

I will need to remove some of the baggage like:

- Scrub any copy about restaurants
- delete Restaurant.elm
- Change Owner.elm -> Manager.elm

client/src/User.elm

@@ -0,0 +1,39 @@
+module User exposing (render)
+
+import Html exposing (..)
+import Html.Attributes exposing (..)
+import Html.Events exposing (..)
+import Maybe.Extra
+import RemoteData
+import State
+import Tailwind
+import UI
+import Utils
+
+
+render : State.Model -> Html State.Msg
+render model =
+    div
+        [ class
+            ([ "container"
+             , "mx-auto"
+             , "text-center"
+             ]
+                |> Tailwind.use
+            )
+        ]
+        [ UI.header 2 ("Welcome, " ++ model.username ++ "!")
+        , UI.simpleButton
+            { label = "Logout"
+            , handleClick = State.AttemptLogout
+            }
+        , case model.logoutError of
+            Nothing ->
+                text ""
+
+            Just e ->
+                UI.errorBanner
+                    { title = "Error logging out"
+                    , body = Utils.explainHttpError e
+                    }
+        ]