Attempt to have less more track variety on submissions 

A simple pie chart to visualize the distribution of session tracks in submissions. Maybe this will help lessen differences.

Secondarily, this pull request also adds a link to the WBMA instructions. 
We're trying to avoid using `Conference.current` as much as possible as singletons are evil (and have already bitten us in the past). I believe we had a before_filter that would set the `@conference` variable. Maybe it's the case of adding it as a general helper and have it default to the last conference for now. Or better even would be to not display the chart unless you have a conference assigned.