Allow to choose from existing tag categories

Also fixes a ui problem with a too long dropdown menu in the small
modal popup when adding tags or custom fields in item detail view.

Issue: #331

modules/webapp/src/main/elm/Comp/DetailEdit.elm

@@ -165,11 +165,11 @@ initTag itemId tm =
     init itemId (TM tm)
 
 
-initTagByName : String -> String -> Model
-initTagByName itemId name =
+initTagByName : String -> String -> List String -> Model
+initTagByName itemId name categories =
     let
         tm =
-            Comp.TagForm.emptyModel
+            Comp.TagForm.emptyModel categories
 
         tm_ =
             { tm | name = name }