We use MfM in all sorts of places, and only some of them are actual
blocks. We can now tell the `Mfm` component to make the top-level
`<bdi>` a block when we need to (mostly note bodies, user
descriptions, announcements) and leave it inline in all other places.
This should still rendener inline rtl content embedded in ltr text in
a sensible way, while providing right-alignment for fully rtl blocks.
43c0ffe7f8