Enable setting bootstrap admins from environment variable

* also update app.yaml.dist to use `BOOTSTRAP_ADMIN_USERS` setting
* set `GGRC_BOOTSTRAP_ADMIN_USERS` in `bin/init_env`
Add `null-if-empty` attribute for context.* hidden inputs

* `null-if-empty` sets input value to `null` if value is empty
* use `null-if-empty` on `context.id` and `context.href` in forms