E-Mail subject rendered without context in E-mail admin list view

[zagl]

In the E-Mail admin list view the subject for pre-rendered emails is rendered again without context.
This is again a direct cause of #403 and similar to issue #421.

The fix would be the same. The check for instance.template in line 134 of admin.py should be replaced with instance.context.

django-post_office/post_office/admin.py

Lines 133 to 135 in 5a1aa53

	def shortened_subject(self, instance):
	if instance.template:
	template_cache_key = '_subject_template_' + str(instance.template_id)