Introduce ui settings and let user set page size for item search

modules/webapp/src/main/elm/Data/UiSettings.elm

@@ -0,0 +1,63 @@
+module Data.UiSettings exposing
+    ( StoredUiSettings
+    , UiSettings
+    , defaults
+    , merge
+    , mergeDefaults
+    , toStoredUiSettings
+    )
+
+{-| Settings for the web ui. All fields should be optional, since it
+is loaded from local storage.
+
+Making fields optional, allows it to evolve without breaking previous
+versions. Also if a user is logged out, an empty object is send to
+force default settings.
+
+-}
+
+
+type alias StoredUiSettings =
+    { itemSearchPageSize : Maybe Int
+    }
+
+
+{-| Settings for the web ui. These fields are all mandatory, since
+there is always a default value.
+
+When loaded from local storage, all optional fields can fallback to a
+default value, converting the StoredUiSettings into a UiSettings.
+
+-}
+type alias UiSettings =
+    { itemSearchPageSize : Int
+    }
+
+
+defaults : UiSettings
+defaults =
+    { itemSearchPageSize = 90
+    }
+
+
+merge : StoredUiSettings -> UiSettings -> UiSettings
+merge given fallback =
+    { itemSearchPageSize =
+        choose given.itemSearchPageSize fallback.itemSearchPageSize
+    }
+
+
+mergeDefaults : StoredUiSettings -> UiSettings
+mergeDefaults given =
+    merge given defaults
+
+
+toStoredUiSettings : UiSettings -> StoredUiSettings
+toStoredUiSettings settings =
+    { itemSearchPageSize = Just settings.itemSearchPageSize
+    }
+
+
+choose : Maybe a -> a -> a
+choose m1 m2 =
+    Maybe.withDefault m2 m1