Check if the link in !news has already been posted to avoid multiple identical posts

[MarcAnt01]

[Arlodotexe]

We can't crawl the entire message history for this -- maybe back 10-15 messages?

We also don't see this happening often, so while we'll approve a PR for this, I have no plans on implementing this myself (unless it becomes a problem).

[sylveon]

Why crawl when you can store the posted links in a database

[Arlodotexe]

Why crawl when you can store the posted links in a database

Changing the database models adds complexity that we can't afford right now. Before we can do that, we must be completely moved to the ASP.NET backend.