Make boards work on standard Matrix clients (Element et al)

[ChurchOfTheSubgenius]

Events in boards use the type space.board.post, which means they don't render on Element or other clients. They also lack a formatted_body in the content as we process/render the markdown in the client.

Here's what would need to change:

• Use regular m.room.message type for board events.
• Add a formatted_body prop to the content, and include the post title in there too so it shows up on other clients.
• Nested threads should at least show up as flat regular Matrix threads.

[ChurchOfTheSubgenius]

One problem with displaying basic versions of Commune boards posts on clients like Element is that we're going to lose custom emojis or any kind of non-basic markdown formatting that we support.

Element is proving to be especially stubborn. I can't for the life of me figure out why it won't render inline images.

I guess we'll just have to render a basic non-formatted post body for legacy matrix clients and cal it a day.