group-pms: Update list when new group pm is made.
group-pms: Update list when new group pm is made.

The function activity.process_loaded_messages(messages) would be called
from message_events.js, this would call people.huddle_string with the
same message object, it was expected that this would return a list of
ids but the message.display_recipient attribute which was being sent
here used a "user_id" field instead of an "id" field.

Fixes: #12503.

brainwane/zulip,eeshangarg/zulip,brainwane/zulip,brainwane/zulip,rishig/zulip,synicalsyntax/zulip,kou/zulip,eeshangarg/zulip,shubhamdhama/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,synicalsyntax/zulip,rht/zulip,andersk/zulip,hackerkid/zulip,brainwane/zulip,showell/zulip,shubhamdhama/zulip,rht/zulip,shubhamdhama/zulip,punchagan/zulip,synicalsyntax/zulip,tommyip/zulip,rishig/zulip,timabbott/zulip,timabbott/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,showell/zulip,hackerkid/zulip,rishig/zulip,punchagan/zulip,timabbott/zulip,zulip/zulip,kou/zulip,tommyip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,kou/zulip,zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,shubhamdhama/zulip,hackerkid/zulip,hackerkid/zulip,showell/zulip,punchagan/zulip,hackerkid/zulip,rishig/zulip,tommyip/zulip,rht/zulip,showell/zulip,andersk/zulip,rishig/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,shubhamdhama/zulip,kou/zulip,eeshangarg/zulip,synicalsyntax/zulip,tommyip/zulip,punchagan/zulip,showell/zulip,timabbott/zulip,eeshangarg/zulip,shubhamdhama/zulip,rishig/zulip,zulip/zulip,shubhamdhama/zulip,brainwane/zulip,punchagan/zulip,showell/zulip,tommyip/zulip,showell/zulip,rishig/zulip,timabbott/zulip,synicalsyntax/zulip,tommyip/zulip,synicalsyntax/zulip,timabbott/zulip,timabbott/zulip,kou/zulip,rht/zulip,punchagan/zulip,kou/zulip,tommyip/zulip,brainwane/zulip,synicalsyntax/zulip,eeshangarg/zulip,brainwane/zulip