Initial implementation of group stats page

This commit adds a public dashboard where you can view stats about your
groups and spending/borrowing habits. Stats such as total exchanged
within the group / total loaned / total borrowed / duration averages are
included.

The implementation uses a group-based partial that gets rendered server-side
and sent back in an ajax call. A delay is simulated.

Charts are not yet implemented - this commit merely wires up the
infrastructure with some preliminary stats.
Merge branch 'summer-features' into stats