Check if page is a donate page through _model field

Part of donate-static#38 (closed) requires removing the html field in templates. This breaks the donate-form pages since header.html checks this.html == 'donate-form.html'. Moving to this._model == 'donate-form' doesn't break anything with the legacy donate layout template, and it continues working with the new template.

This will block a future MR (not yet sent, but ready to send) to tpo/web/donate-static since changing the donate-form pages relies on this MR.