[FIX] mail: click on unstar all button should reset the starredcounter

Before this commit:
Clicking on 'unstar all' button removes all messages from starred channel
but it only decreases the counter as many as there are displayed messages
on screen.

For example, if there are 40 messages starred, only 30 messages
are on screen (default limit) but unstar all will decrease counter
only for those messages instead of 40.

After this commit:
clicking on 'unstar all' button will correctly take in account all
fetched messages and not only displayed messages.
So now, after clicking on 'unstar all' button will update the starred
counter to 0.

task-ID: 33958