Add a message level dict to `contrib.messages.context_processors.messages`

[Baptiste Mispelon]

It'd be nice if the provided context processor also added a dictionary of message levels to the context.

This would allow something like this:

{% for message in messages %}
    {% if message.level == MESSAGE_LEVELS.SUCCESS %}...{% endif %}
    ...
{% endfor %}

Currently, one has to pass the message levels manually into the context, or do something like this, which is brittle:

{% if 'success' i nmessage.tags %}...{% endif %}

There should not be any backwards-compatibility issue and the change is rather minimal.
I've attached a proof-of-concept patch. If there's interest, I'd be willing to polish it up (mostly adding tests and documentation).

[Aymeric Augustin]

Technically, it's backwards incompatible for people who have "MESSAGE_LEVELS" in their context. That's minor, but it should be mentioned in the release notes.

[Baptiste Mispelon]

Here's a pull request with tests and documentation: ​https://github.com/django/django/pull/1822

[Baptiste Mispelon <bmispelon@…>]

In 9fde42a69a56cb6c8cdfd67b07c67a1b97638f12:

Fixed #20610: Added a message level dict to contrib.message context processor.

[Claude Paroz <claude@…>]

In 79e4a189f3b17b5a7426f889c4412126eb169256:

[1.7.x] Updated the messages context processor docs

Refs #20610.
Backport of 7951f125e frmo master.