Mouse over for cover images

[teemu]

When putting a mouse over the cover image the "image will turn around" and show:

• By: "authors name"
• Add to LeMill by: "name here"
• "First 80 characters of the textual content"

The idea is that it should looks like a "backcover of a book". It would be nice if it really changes the whole image and displays the text on a white background, but we may go with more simple "link title text" / "tip text", as well.

The title text is always visible below the cover image like in section front pages.

[tarmo]

This should probably also show tags? Since apparently we're not going to show any other information in listings that what's here.

[tarmo]

Implemented, and then cancelled.