Use Django proxy models for markdown extensions of Message

[timabbott]

We have several extra fields we add to Django Message objects when doing Bugdown rendering (e.g. potential_attachment_urls, mentions_wildcard, etc.). Currently, it's a bit of a hack, in that these values we attach to the Message object aren't part of its class definition..

One reasonable approach for typing this flow (enforced to some extent by how python-markdown is architected) would be to define a BugdownMessage class using Django's Proxy Models feature (https://docs.djangoproject.com/en/3.0/topics/db/models/#proxy-models).

While we're at it, _md_engine.zulip_db_data could probably be made a TypedDict so it can be typed nicely.

@neiljp FYI

[zulipbot]

Hello @zulip/server-markdown, @zulip/server-refactoring members, this issue was labeled with the "area: markdown", "area: refactoring" labels, so you may want to check it out!

[EydenVillanueva]

Hello, is anyone working on this? Can I claim it ?

[Kushal-kothari]

@zulipbot claim

[zulipbot]

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