Shake up the about pages a bit

 - move details and contact to their own pages
 - add contact to footer
 - change "gift" nomenclature to "payment"
 - add "example" and "where next" sections to /about
Fix test suite
Support /charts in addition to /charts.html
Clean up details page
Standardize on foo.spt under about/
Change Gittip to Gratipay in legalese
Change a couple more Gittips to Gratipay

While we're at it ...
add missing speclines
Merge pull request #2803 from gratipay/refactor-about

Shake up the about pages a bit