Improve Task Volumes page look

[RodionGork]

Now tasks are categorized and "Problem" link leads to the page listing "problem volumes". Current simple list of links should be improved so that every link is enclosed in larger rectangular tile (like how it is done at http://rosalind.info/problems/locations/). We probably shall have small description for each of them later - and, probably, counts of tasks.

[RodionGork]

That's done