Remove message.is_mentioned in message events.

This field would get overwritten with an improper value when
we looped over multiple clients, due to not making full copies
of the message dictionary.  This failure would be somewhat
random depending on how clients were ordered in the loop.

The only consumers of this field were the mobile app and the
apply-events-to-unread-counts logic.  Both of these will now
use `flags` instead.

zerver/tests/test_events.py

@@ -554,7 +554,6 @@ class EventsRegisterTest(ZulipTestCase):
                 ('content', check_string),
                 ('content_type', equals('text/html')),
                 ('display_recipient', check_string),
-                ('is_mentioned', check_bool),
                 ('is_me_message', check_bool),
                 ('reactions', check_list(None)),
                 ('recipient_id', check_int),

zerver/tornado/event_queue.py

@@ -779,8 +779,6 @@ def process_message_event(event_template, users):
             message_dict = message_dict.copy()
             message_dict["invite_only_stream"] = True
 
-        if flags is not None:
-            message_dict['is_mentioned'] = 'mentioned' in flags
         user_event = dict(type='message', message=message_dict, flags=flags)  # type: Dict[str, Any]
         if extra_data is not None:
             user_event.update(extra_data)