62aca1e976
this is an undocumented room state event that at least Element uses to disable/enable URL previews by default for the entire room. we'd like to use this to disable URL previews in the conduwuit admin room where we have large message blocks with some text that clients may render as URLs, or a list of domains from the server config output. also see https://github.com/matrix-org/matrix-spec/issues/394 Signed-off-by: strawberry <strawberry@puppygock.gay>