When calculating entry ratings, make decay of access value dependent on usage frequency

[hnicke]

Witnessed behaviour

An access loses value over time. After ~20 days, the access is almost worth nothing. If the user doesn't use his shell for 20 days (vacation, illness etc), all ratings are basically the same.

Expected behaviour

The ratings should not decay if there is no usage at all. I.e., we should not only use an entries age to calculate its value.