Prepare for selecting languages

UI language is stored in user settings for authenticated users;
otherwise is only stored in the current model (not persisted).

modules/webapp/src/main/elm/Messages/App.elm

@@ -0,0 +1,22 @@
+module Messages.App exposing
+    ( Texts
+    , de
+    , gb
+    )
+
+
+type alias Texts =
+    { login : String
+    }
+
+
+gb : Texts
+gb =
+    { login = "Login"
+    }
+
+
+de : Texts
+de =
+    { login = "Anmelden"
+    }