Weight most downloads/liked plugins by last update

[mrclay]

If age is more than say, 2 years old, age should start to outweigh the other factor(s).

This expression yields 1 until age 2 then decreases to 0 at age 5:

years_old = (UNIX_TIMESTAMP() - e.last_action) / 31556900
freshness = (3 - MIN(3, MAX(0, years_old - 2))) / 3;
= (3 - MIN(3, MAX(0, (UNIX_TIMESTAMP() - e.last_action) / 31556900 - 2))) / 3

Small problems: we don't touch the last_action of the parent project when adding a release (I think). Obviously we'll have to cache this ordering.