Bug fix: display form errors when creating new tenant

in case of form validation errors the page is displayed again but
previous implementation was hiding the error(s) b/c it was creating
a new form instance unconditionally.

With this chenge if a form instance already exists, possibly
containing errors, it will be reused. Otherwise we default to creating
a new form instance. The existing test should report PASS.

tcms_github_marketplace/views.py

@@ -284,7 +284,7 @@ def get_context_data(self, **kwargs):
             self.purchase.effective_date)
 
         context = super().get_context_data(**kwargs)
-        context['form'] = context['form'].__class__(
+        context['form'] = kwargs.get('form') or context['form'].__class__(
             initial={
                 'on_trial': False,
                 'paid_until': paid_until,