make GEXF files downloadable

 - views/urls for serving out GEXF

 - link from network overview and both BG network graph pages

[#83695266]
adjust edge weights for groupsheet network model
adjust d3.js force graph to allow adjusting link weight/stroke width
tweak network graph settings for groupsheet model display
re-organize stories in changelog by functionality

[#83853038]
update changelog with stories from pivotal tracker since sept. 2013

[#83853038]