admin: use gisadmin everywhere

gisadmin automatically falls back to the django.contrib.admin anyway.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
models: update value to 1024-char field

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
views, urls: plan out various API endpoints

Get ^id/(\d+)$ to return JSON, and build wireframes of the others.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>