Make top header into a link to /about/charts.html.

First task on #1401.
Replace 'Funders' with 'Active Users' on charts.

Second and third task on #1401.
Shuffle graphs around to line them up better.
Adjust whitespace.
Tweak "per week" on charts

I had specified ", per week" on the ticket, but then changed it (pretty
quickly) to " (per week)." @zwn was perhaps working from the email and
didn't catch the change.
Merge pull request #1403 from gittip/issue-1401

Set header to link to /about/stats.html and replace Funders chart with Active Users.