Newsfeed uses pure-css grid

Pure's css grid can be full width without causing an extraneous horizontal scroll bar through negative margins.

A media query has also been added so that the chat stream disappears if the screen is too narrow.
YouTube videos are embedded directly in the newsfeed

This is done through a de-render-entry-stream directive.  At the moment, YouTube videos are the only ones that have a non-default partial template for this, but there are more to come.
ContentController returns the JSON of the entry

Previously there had been an error where editItem was returning it as { entry: json } and editTags was returning it as { course: json }.

This commit also makes the MarkdownPage view controller watch for changes to the item text, rather than just rendering once when it is first instantiated.

Between them, this fixes some buggy editing behaviour for content entries.
Added support for committing Markdown pages as "news posts"