Allow to select multiple items for deletion and edit

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

@@ -0,0 +1,32 @@
+module Data.ItemSelection exposing
+    ( ItemSelection(..)
+    , isActive
+    , isSelected
+    )
+
+import Set exposing (Set)
+
+
+type ItemSelection
+    = Inactive
+    | Active (Set String)
+
+
+isSelected : String -> ItemSelection -> Bool
+isSelected id set =
+    case set of
+        Inactive ->
+            False
+
+        Active ids ->
+            Set.member id ids
+
+
+isActive : ItemSelection -> Bool
+isActive sel =
+    case sel of
+        Active _ ->
+            True
+
+        Inactive ->
+            False