This adds support for unstarring all (starred)
messages from a particular topic, from the topic
popover.
The earlier implementation of this in #16898
was reverted in bc55aa6a01 (#17429)
because it had two problems-
1. The crash reported in bc55aa6a01
was due to message_store returning undefined. This happens
when the message itself hasn't been fetched from the server
yet, but we know that the message is starred from the ids
in `page_params` in `starred_messages.js`.
This commit handles this case explicitly.
Note that, we simply ignore those messages which
we haven't fetched, and because of this, it may
happen that we don't unstar some messages from that
topic. The correct implementation for this would
be to ask the backend for starred IDs in a topic.
2. The earlier implementation actually unstarred **all**
messages. This was because it grabbed the topic and stream_id
from the topic popover `data` attributes, after the topic
popover had been closed. This passed `undefined`, which
the function then interpreted as an action to unstar all
messages.
With this commit, we use the confirm_dialog widget,
which eliminates the need to store this data in the DOM.
Abhijeet Prasad Bodas