Added support for Mermaid and PlantUML, and fixed some bugs in the admin panel

Short overview of things were done

While writing a post, I needed to insert diagrams to illustrate a certain concept (here). As I was writing, I realized I didn't have diagram rendering support for any type of post, which gave me the idea to add a renderer. Of course, it didn't end with just Mermaid; I also decided to add PlantUML, which, it turns out, doesn't have a public API. I had to set up a local service on the server and run renders from it.

Also, I realized that screenshot testing wasn't working quite correctly because, while it tests some public pages via mocks, it doesn't test all public reusable components. I had to add support for that as well.

Then I continued writing and realized there was no support for bullet points, but fortunately, Blocknote supports them out of the box, so I didn't have to build a bunch of them.