Place video description in block element

The summary is passed to markdown, which will produce at least one
block-level element: a paragraph.
Since block elements are not allowed inside inline elements, change the
tag to div.
250. Constrain large images in video summary/description

Images might be larger than the summary/description container and will
stick out on the right. With this change images can be at most the width
of the container they are in.

Fixes: #250
Merge pull request #257 from squiddy/250-constraint-images

Constrain large images in video summary/description