Make the teaser block always on top

* The teaser block will be moved to the first position no matter where it
was created.
* Moving the teaser block using the Reorder will set it back to position 1
* Moving other block using the Reorder to position 1 will just move it to
position 2 instead
* Because we have to tab, the markdown block's background can just
be transparent to look like a content editable div like the rest.
* Comment is now a Markdown block.

@TODO: Maybe just move the other block to where it was?
@TODO: Support dragging to change order too.