Add default/fallback behavior for subscription category with 'Cannot unsubscribe' checked

[cjg89]

Currently, if you create a subscription category with the 'Cannot unsubscribe' checkbox checked, but no 'Applies to' value, the front-facing recipient subscriptions view will throw an error due to attempting to match the user's email address against an invalid regex pattern. This also occurs if 'Applies to' is set to *.

PostMaster/templates/manager/recipient-subscriptions.html

Line 24 in 1323ec5

{% if category|can_unsubscribe:recipient.email_address %}

PostMaster/manager/templatetags/filters.py

Lines 32 to 37 in 1323ec5

	@register.filter
	def can_unsubscribe(category, email_address):
	if category.cannot_unsubscribe and category.applies_to_email(email_address):
	return False
	return True

https://github.com/UCF/PostMaster/blob/master/manager/models.py#L164-L168

It would be nice if the 'Applies to' value could be empty or * and prevent unsubscribes for all email addresses. Alternatively, if that's not desirable, we could show/hide the 'Applies to' field with some basic js logic depending on if 'Cannot unsubscribe' is checked, and/or add a field description that mentions the 'Applies to' field being required and that a wildcard * is not supported.