Refactor groups index page

[zephod]

Groups are listed alphabetically with paging - not an ideal user experience. We would like to list groups in order of 'popularity': The number of datasets they contain.

Following this chain of thought, then, it would be nice to rearrange the groups table by clicking on column headers and having it sort by that column.

Furthermore, then, we'd like to implement a full-fledged groups search feature (if this is at all feasible).

The forthcoming groups refactor will probably have some bearing on this task.