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/user_pill.js

@@ -22,6 +22,7 @@ export function create_item_from_email(email, current_items) {
             // so we just create a pill where the display value
             // is the email itself.
             return {
+                type: "user",
                 display_value: email,
                 email,
             };
@@ -42,6 +43,7 @@ export function create_item_from_email(email, current_items) {
     // We must supply display_value for the widget to work.  Everything
     // else is for our own use in callbacks.
     const item = {
+        type: "user",
         display_value: user.full_name,
         user_id: user.user_id,
         email: user.email,
@@ -61,6 +63,7 @@ export function append_person(opts) {
     const avatar_url = people.small_avatar_url_for_person(person);
 
     pill_widget.appendValidatedData({
+        type: "user",
         display_value: person.full_name,
         user_id: person.user_id,
         email: person.email,