Notifications: filter out cancelled and dismissed from templates

[humitos]

We are always displaying all notifications to users, but we should only show not cancelled or dismissed:

readthedocs.org/readthedocs/templates/core/project_bar_base.html

Lines 37 to 41 in 8eda981

	{% for notification in project.notifications.all %}
	<p class="build-failure">
	{{ notification.get_message.get_rendered_body|safe }}
	</p>
	{% endfor %}

We should probably create a QuerySet method for this called display or show so we call as project.notifications.display().