Notifications: migrate old custom Django message notifications

[humitos]

I kept the rendering for the default Django messages in the base template. However, we need to take a deeper look at it since it used to use django-messages-extends for extra and custom notifications (those that can be dismissed, etc) that we are not using anymore. That means the "Mark as read" URL and others won't work anymore.

Those messages have to be split into two groups:

1. regular Django messages attached to the request
2. notifications attached to the user that can be dismissed

readthedocs.org/readthedocs/templates/base.html

Lines 121 to 139 in 7d1daee

	<!-- BEGIN notify -->
	{% block notify %}
	{# TODO: migrate these notifications to new system #}
	{% if messages %}
	<ul class="notifications">
	{% for message in messages %}
	<li class="notification notification-{{ message.level }}" {% if message.pk %}data-dismiss-url="{% url 'message_mark_read' message.pk %}{% endif %}">
	{% if message.pk %}
	<a class="notification-action" href="{% url 'message_mark_read' message.pk %}">
	<span class="icon close" aria-label="{% trans 'Close notification' %}"></span>
	</a>
	{% endif %}
	{{ message }}
	</li>
	{% endfor %}
	</ul>
	{% endif %}
	{% endblock %}
	<!-- END notify -->

We talked about this on the PR while working on this: #10922 (comment)

[agjohnson]

If I understand correctly, our plan on the old dashboard should be:

• Anything left using Django messages at this point won't be changed to our new notification backend. These are form confirmations and other temporary Django messages.
• We'd keep the template logic above for standard Django messages. The template is actually still accurate for standard messages (not sticky/persistent messages from messages-extends).
• New Notification instances should use HTML similar to this template instead of separate HTML.

The new dashboard handles standard Django messages separately from sticky messages:

https://github.com/readthedocs/ext-theme/blob/bff4b4ddf645c9e5f690dd086f47602a74e7a429/readthedocsext/theme/templates/includes/utils/messages.html#L11-L45

That logic will need to be changed to:

• Iterate over notifications to show Notification instances. The template logic will be changed slightly to accommodate the new Message class.
• Iterate over messages to continue showing Django messages as toast messages. This logic doesn't change at all.

[humitos]

Yes, I understand the summary of the plan you wrote is 👍🏼

The code will be simplified since message.pk won't exist anymore since there is no messages saved into the database anymore since we stopped used sticky/permanent messages via django-messages-extends.

Those sticky/permanent messages are now Notification objects attached to a particular resource. The code to render all of these does not exist yet and part of the work required for this issue.