This new site setting determines the maximum age of unread topics in
suggested. By default if you have any unread topics older than 90 days
they will be omitted from suggested.
This change was added for 2 reasons:
1. A performance safeguard, some users tend to collect a huge amount of
read state so it becomes super expensive to find unread
2. People who collect a large amount of unread are much more interested in
recent unread topics vs ancient unread topics, this makes suggested more
relevant
Also, this is a minor speed up for tests cause 3 expensive tests became 1.
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