Image drop, teaser block on load, tweaks on code block

* Markdown block supports drag and drop image to it, which will insert
an `<img />` at the caret and display the image in preview.
* If the entry already has teaser, a teaser block will be displayed.
* The teaser is just a text block now.
* Trim the newlines before and after the code block during parsing.
* Move the filename to the UI of the code block.
* Add an option to un-embed a code block in the UI.