js: Use IntDict in people.js.

This required lots of manual testing:

    - search/navigate user presence
    - send PM and mention user
    - pay attention to compose fade
    - send stream msg and mention user
    - open Private Messages in top-left and click
    - test unread counts
    - invite user who already has account
    - search for users in search bar
    - check user settings
        - User Groups
        - Users
        - Deactivated Users
        - Bots
    - create a bot
    - mention user groups
    - send group PM then click on lower right
    - view/edit/create streams

If there are still pieces of code that don't convert
ids to ints, the code should still work but report
blueslip errors.

I try to mostly convert user_ids to ints in the callers,
since often the callers are dealing with small amounts
of data, like user ids from huddles.

static/js/message_list_view.js

@@ -30,7 +30,7 @@ function get_user_id_for_mention_button(elem) {
     }
 
     if (user_id) {
-        return user_id;
+        return parseInt(user_id, 10);
     }
 
     if (email) {