New organization and contact forms (inc. for sign-up)

For new users, existing users and admin users.

Includes:

* Significant DRYing/deduplication of form definition and
  population of objects (using model_form() for GAE)
* Generation of form HTML where standard (using WTForms)
* Some refactoring of POST handling in the admin interface
  to use the same handler classes as GET requests
  (instead of the general admin_handler)
* Template macros (Jinja2), currently stored under /admin_handler/