Add time-based cutoffs for public tag cloud, favorited lists to speed up those queries.

Add time-based cutoffs for public tag cloud, favorited lists to speed up those queries.
Defaulting to only looking at last 90 days of activity, can be adjusted up or down.

$config['tag']['cutoff'] = 86400 * 90;
$config['popular']['cutoff'] = 86400 * 90;

Per-user and per-group tag clouds do not use the cutoff (and it doesn't help with indexing on them).