Add support for Mention

[bytrangle]

If I mention a page, for example its title is "A Real World Article", it will be just plain text in the markdownString.

What I think should be rendered:

[A Real World Article](/a1b2c3d4)

where a1b2c3d4.

What do you think?

[orlowdev]

Yeah, I thought about it. But the problem is when you mention a page, it doesn't necessarily mean that that the mentioned page is available with current credentials. This can be checked per page, but it would require downloading too much stuff from the Notion API.