Previously, this feature was disabled by default. However, despite being
one of the best features on the site, only about 10% of users ever
enabled it, and even very involved/frequent users often didn't realize
it existed.
My original thought about why it should be opt-in only is that I thought
it had a meaningful privacy impact, but it really doesn't. User visits
to topics are already tracked through server logs and similar data, so
the feature doesn't really make any difference.
This commit enables the feature for everyone, removes the separate
Settings page, and moves the "Collapse old comments" sub-setting onto
the main Settings page.