refactor: Add type field to pill items.

Item field is added to the pill items, this is done to utilize
it wherever possible, to distinguish between diffrent types
of pill items(stream|user_groups|users).

It is also a preparatory commit to support expanding of stream
and user group pills.

static/js/input_pill.js

@@ -94,6 +94,11 @@ export function create(opts) {
                 return;
             }
 
+            if (!item.type) {
+                blueslip.error("no type defined for the item");
+                return;
+            }
+
             const payload = {
                 id,
                 item,