Cover images in section front page are displayed according to usage information of resources

[hans]

Cover images in section front page are displayed according to usage information of resources (#427):

• the status of the resource must be published
• the content must be published or edited during last 30 days (for activities and tools 90 days)
• materials should have higher ranking in the algoritm than pieces
• resources that are included in the collection have higher ranking
• ...

[hans]

Please use this story to develop and discuss the algoritm for displaying cover images.

[tarmo]

To include amortization, I'd decrease the weight of older events. Meaning that any views of an item would have the most value (1) when they're recent, and have less and less value as they get older (for example, when over 1 year old, not counted at all).