Pre-renders markdown to HTML on save

Instead of firing up the renderer each page load, it adds two new columns to
Item: html, which contains the entire html output, and summary, which contains
the output from the first 200 chars of the text. Multiple columns are required
because just chopping the HTML at an arbitrary length would result in a broken
tag. And I'd rather not detect and close the tags on each render of an item.
Increases preview length to 500ch, adds snippet