Make overview page display one data point per week

[OsamaSayegh]

Currently the overview page (e.g. /ruby/ruby/commits/overview) tries to display a graph with 2000 data points (BenchmarkRun) for every BenchmarkType. This is very expensive and takes the page about 5 minutes to load on my local instance.

This PR changes the overview page so it picks a single commit per week and displays the data point associated with that commit. The page now takes 1.5 seconds to load locally without cache. We already have a 30 minutes cache in place for the page, so we don't have to do the work repeatedly.

Screenshot:

I also added a link to the overview page on the /benchmarks page:

[OsamaSayegh]

@tgxworld can you review this whenever you get a chance? thanks! ^_^