support: Always show the option to update end_date on /support page.

If a plan is already on "basic" or "business" plan and wants to
switch to a fixed-price "basic" / "business" plan, then it is
necessary that the current plan should have an end date configured.

Earlier, we could update the end_date only if the current plan
already had an end_date set.

This commit makes it possible to always show the option to
set or update end_date.

corporate/lib/stripe.py

@@ -1312,7 +1312,6 @@ class BillingSession(ABC):
         if customer is not None:
             plan = get_current_plan_by_customer(customer)
             if plan is not None:
-                assert plan.end_date is not None
                 assert plan.status == CustomerPlan.ACTIVE
                 old_end_date = plan.end_date
                 plan.end_date = new_end_date

templates/corporate/support/current_plan_forms_support.html

@@ -9,12 +9,13 @@
     <button type="submit" class="support-submit-button">Update</button>
 </form>
 
-{% if current_plan.end_date and current_plan.status == current_plan.ACTIVE %}
+{% if current_plan.status == current_plan.ACTIVE %}
 <form method="POST" class="remote-form">
     <b>Plan end date</b><br />
     {{ csrf_input }}
     <input type="hidden" name="{{ remote_type }}" value="{{ remote_id }}" />
-    <input type="date" name="plan_end_date" value="{{ current_plan.end_date.strftime('%Y-%m-%d') }}" required />
+    <input type="date" name="plan_end_date" {% if current_plan.end_date %}
+      value="{{ current_plan.end_date.strftime('%Y-%m-%d') }}" {% endif %} required />
     <button type="submit" class="support-submit-button">Update</button>
 </form>
 {% endif %}