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/Manager.elm

@@ -0,0 +1,46 @@
+module Manager exposing (render)
+
+import Array
+import Html exposing (..)
+import Html.Attributes exposing (..)
+import Html.Events exposing (..)
+import RemoteData
+import State
+import Tailwind
+import UI
+import Utils
+
+
+render : State.Model -> Html State.Msg
+render model =
+    case model.session of
+        Nothing ->
+            text "You are unauthorized to view this page."
+
+        Just session ->
+            div
+                [ class
+                    ([ "container"
+                     , "mx-auto"
+                     , "text-center"
+                     ]
+                        |> Tailwind.use
+                    )
+                ]
+                [ h1 []
+                    [ UI.header 2 ("Welcome back, " ++ session.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
+                                }
+                    ]
+                ]