Support free plans.

Fixes #24 (https://github.com/cypreess/django-plans/issues/24)

Plans without pricings will be `plan.is_free: true`. Switching to these
plans causes the userplan have it's expiry cleared.

Since there's no expiry set, the standard plan change process doesn't
make sense, so the plan table is adjusted to present the standard plan
order links rather than plan change links.

Signed-off-by: Devang Shah <shahd@vmware.com>