Display attribution & License text at the bottom of each page

Also has a bunch of other major changes:

- Changed the way how ApiTask works, to make it possible for
  child classes to override the params being sent to the server
- Added a PageProperties object that contains properties related
  to a page. Currently only last modified time, but infrastructure
  exists to make it have more things in the future.
- Fix bug in external link handling that tried to open all mediawiki
  pages, rather than just wiki pages, internally
- Add functions for for relative formatting of time

Change-Id: I713e17e98bf83dc7f4dd89f6d65254405cfaa390